Cap ed25519-dalek <3.0.0 to keep it aligned with rand 0.8.5

ed25519-dalek 3.0.0 (2026-07-06) bumped its rand_core past the rand 0.8.5 /
rand_chacha 0.3.1 pinned in soroban-env-host, breaking the ChaCha20Rng:
CryptoRng bound used by SigningKey::generate in builtin_contracts testutils.
Cap the requirement at <3.0.0 so fresh downstream resolves (e.g. the SDK's
cargo-semver-checks rustdoc build) pick 2.x.

Author: sisuresh

soroban-env-host/Cargo.toml

@@ -24,7 +24,9 @@ sha2 = "0.10.8"
 hex-literal = "0.4.1"
 hmac = "0.12.1"
 # NB: We'll need to pin this again after switching the Core to the new env version.
-ed25519-dalek = {version = ">=2.0.0", features = ["rand_core"] }
+# ed25519-dalek 3.0.0 (2026-07-06) bumped its rand_core past the rand 0.8.5 /
+# rand_chacha 0.3.1 pinned below, breaking `ChaCha20Rng: CryptoRng`. Cap <3.0.0.
+ed25519-dalek = {version = ">=2.0.0, <3.0.0", features = ["rand_core"] }
 # NB: this must match the same rand version used by ed25519-dalek above
 rand = "0.8.5"
 # NB: this must match the same rand_chacha version used by ed25519-dalek above