Bump version to 25.3.0 (#1751)

### What
Bump version to 25.3.0, creating release branch.

### Why
Triggered by @jayz22 in
https://github.com/stellar/rs-soroban-sdk/actions/runs/22787639472.

### 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/v25.3.0` 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=v25.3.0&title=25.3.0

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=v25.3.0&title=25.3.0&target=release/v25.3.0

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Jay Geng <jay@stellar.org>

Author: github-actions[bot]

.github/workflows/rust.yml

@@ -302,7 +302,7 @@ jobs:
         sys:
         - os: ubuntu-latest
           target: wasm32v1-none
-          cargo-hack-feature-options: --manifest-path Cargo.toml --exclude soroban-meta --exclude soroban-spec --exclude soroban-spec-rust --exclude soroban-ledger-snapshot --exclude-features testutils,docs,default,std --feature-powerset
+          cargo-hack-feature-options: --manifest-path Cargo.toml --exclude soroban-meta --exclude soroban-spec --exclude soroban-spec-rust --exclude soroban-ledger-snapshot --exclude-features testutils,docs,default,std,experimental_spec_shaking_v2 --feature-powerset
         - os: ubuntu-latest
           target: x86_64-unknown-linux-gnu
           cargo-hack-feature-options: '--feature-powerset --exclude-features docs'