Bump version to 20.3.1 (#1224)

### What
Bump version to 20.3.1, creating release branch.

### Why
Triggered by @leighmcculloch in
https://github.com/stellar/rs-soroban-sdk/actions/runs/7792756202.

### What is next

See the release instructions for a full rundown on the release process:
https://github.com/stellar/actions/blob/main/README-rust-release.md

Commit any changes to the `release/v20.3.1` branch that are needed in
this release.

If this is a regular release releasing from `main`, merge this PR when
ready, and after merging, create a release for this version by going to
this link:
https://github.com/stellar/rs-soroban-sdk/releases/new?tag=v20.3.1&title=20.3.1

If this is a backport or patch release of a past version, see the
release instructions. When ready to release this branch create a release
by going to this link:

https://github.com/stellar/rs-soroban-sdk/releases/new?tag=v20.3.1&title=20.3.1&target=release/v20.3.1

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Leigh McCulloch <351529+leighmcculloch@users.noreply.github.com>

Author: github-actions[bot]

.github/workflows/rust.yml

@@ -136,7 +136,7 @@ jobs:
     steps:
     - uses: actions/checkout@v3
     - uses: stellar/actions/rust-cache@main
-    - run: rustup install nightly
+    - run: rustup install nightly-2024-02-03
     - uses: stellar/binaries@v15
       with:
         name: cargo-fuzz
@@ -148,15 +148,16 @@ jobs:
     steps:
     - uses: actions/checkout@v3
     - uses: stellar/actions/rust-cache@main
-    - run: rustup install nightly
+    # TODO: Upgrade to latest nightly after problem that was introduced in nightly-2024-02-05 (https://github.com/dalek-cryptography/curve25519-dalek/issues/618) is resolved.
+    - run: rustup install nightly-2024-02-03
     - run: make doc
 
   readme:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
     - uses: stellar/actions/rust-cache@main
-    - run: rustup install nightly
+    - run: rustup install nightly-2024-02-03
     - run: make readme
     - run: git add -N . && git diff HEAD --exit-code
 

Cargo.lock

@@ -29,16 +29,16 @@ members = [
 ]
 
 [workspace.package]
-version = "20.3.0"
+version = "20.3.1"
 rust-version = "1.74.0"
 
 [workspace.dependencies]
-soroban-sdk = { version = "20.3.0", path = "soroban-sdk" }
-soroban-sdk-macros = { version = "20.3.0", path = "soroban-sdk-macros" }
-soroban-spec = { version = "20.3.0", path = "soroban-spec" }
-soroban-spec-rust = { version = "20.3.0", path = "soroban-spec-rust" }
-soroban-ledger-snapshot = { version = "20.3.0", path = "soroban-ledger-snapshot" }
-soroban-token-sdk = { version = "20.3.0", path = "soroban-token-sdk" }
+soroban-sdk = { version = "20.3.1", path = "soroban-sdk" }
+soroban-sdk-macros = { version = "20.3.1", path = "soroban-sdk-macros" }
+soroban-spec = { version = "20.3.1", path = "soroban-spec" }
+soroban-spec-rust = { version = "20.3.1", path = "soroban-spec-rust" }
+soroban-ledger-snapshot = { version = "20.3.1", path = "soroban-ledger-snapshot" }
+soroban-token-sdk = { version = "20.3.1", path = "soroban-token-sdk" }
 
 [workspace.dependencies.soroban-env-common]
 version = "=20.2.1"

Cargo.toml

@@ -6,7 +6,8 @@ CARGO_DOC_ARGS?=--open
 
 doc: fmt
 	cargo test --doc -p soroban-sdk -p soroban-sdk-macros --features testutils
-	cargo +nightly doc -p soroban-sdk --no-deps --features docs,testutils $(CARGO_DOC_ARGS)
+    # TODO: Upgrade to latest nightly after problem that was introduced in nightly-2024-02-05 (https://github.com/dalek-cryptography/curve25519-dalek/issues/618) is resolved.
+	cargo +nightly-2024-02-03 doc -p soroban-sdk --no-deps --features docs,testutils $(CARGO_DOC_ARGS)
 
 test: fmt build
 	cargo hack --feature-powerset --ignore-unknown-features --features testutils --exclude-features docs test
@@ -23,11 +24,11 @@ check: build fmt
 	cargo hack check --release --target wasm32-unknown-unknown
 
 build-fuzz:
-	cd tests/fuzz/fuzz && cargo +nightly fuzz check
+	cd tests/fuzz/fuzz && cargo +nightly-2024-02-03 fuzz check
 
 readme:
 	cd soroban-sdk \
-		&& cargo +nightly rustdoc -- -Zunstable-options -wjson \
+		&& cargo +nightly-2024-02-03 rustdoc -- -Zunstable-options -wjson \
 		&& cat ../target/doc/soroban_sdk.json \
 		| jq -r '.index[.root].docs' \
 		> README.md
@@ -36,7 +37,7 @@ watch:
 	cargo watch --clear --watch-when-idle --shell '$(MAKE)'
 
 watch-doc:
-	cargo +nightly watch --clear --watch-when-idle --shell '$(MAKE) doc CARGO_DOC_ARGS='
+	cargo +nightly-2024-02-03 watch --clear --watch-when-idle --shell '$(MAKE) doc CARGO_DOC_ARGS='
 
 fmt:
 	cargo fmt --all