Bump version to 25.3.1 (#1821)

### What
Bump version to 25.3.1, creating release branch.

### Why
Triggered by @mootz12 in
https://github.com/stellar/rs-soroban-sdk/actions/runs/24035155761.

### 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/v25.3.1` 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=v25.3.1&title=25.3.1

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=v25.3.1&title=25.3.1&target=release/v25.3.1

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

Author: github-actions[bot]