Bump version to 27.0.0 (#1925)

### What
Bump version to 27.0.0, creating release branch.

### Why
Triggered by @mootz12 in
https://github.com/stellar/rs-soroban-sdk/actions/runs/28944621585.

### 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/v27.0.0` 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=v27.0.0&title=27.0.0

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=v27.0.0&title=27.0.0&target=release/v27.0.0

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: mootz12 <mootz12@protonmail.com>
Co-authored-by: mootz12 <38118608+mootz12@users.noreply.github.com>

Author: github-actions[bot]

.github/workflows/rust.yml

@@ -97,6 +97,9 @@ jobs:
     - uses: stellar/actions/rust-check-git-rev-deps@main
 
   semver-checks:
+    # When a PR enters the merge queue, the event context changes from pull_request to merge_group,
+    # so github.event.pull_request becomes null/empty, so we don't do this check in the merge queue.
+    if: "!contains(github.event.pull_request.labels.*.name, 'skip-semver-checks') && github.event_name != 'merge_group'"
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3