Bump version to 20.3.0 (#1365)

### What
Bump version to 20.3.0, creating release branch.

### Why
Triggered by @dmkozh in
https://github.com/stellar/rs-soroban-env/actions/runs/8300327197.

### 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/v20.3.0` 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-env/releases/new?tag=v20.3.0&title=20.3.0

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-env/releases/new?tag=v20.3.0&title=20.3.0&target=release/v20.3.0

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

Cargo.toml

@@ -20,15 +20,15 @@ exclude = ["soroban-test-wasms/wasm-workspace"]
 # NB: When bumping the major version make sure to clean up the
 # code guarded by `unstable-*` features to make it enabled
 # unconditionally.
-version = "20.2.2"
+version = "20.3.0"
 rust-version = "1.74.0"
 
 [workspace.dependencies]
-soroban-env-common = { version = "=20.2.2", path = "soroban-env-common", default-features = false }
-soroban-env-guest = { version = "=20.2.2", path = "soroban-env-guest" }
-soroban-env-host = { version = "=20.2.2", path = "soroban-env-host" }
-soroban-env-macros = { version = "=20.2.2", path = "soroban-env-macros" }
-soroban-builtin-sdk-macros = { version = "=20.2.2", path = "soroban-builtin-sdk-macros" }
+soroban-env-common = { version = "=20.3.0", path = "soroban-env-common", default-features = false }
+soroban-env-guest = { version = "=20.3.0", path = "soroban-env-guest" }
+soroban-env-host = { version = "=20.3.0", path = "soroban-env-host" }
+soroban-env-macros = { version = "=20.3.0", path = "soroban-env-macros" }
+soroban-builtin-sdk-macros = { version = "=20.3.0", path = "soroban-builtin-sdk-macros" }
 
 # NB: When updating, also update the version in rs-soroban-env dev-dependencies
 [workspace.dependencies.stellar-xdr]