Bump version to 21.6.0 (#1325)

### What
Bump version to 21.6.0, creating release branch.

### Why
Triggered by @leighmcculloch in
https://github.com/stellar/rs-soroban-sdk/actions/runs/10481503334.

### 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/v21.6.0` 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=v21.6.0&title=21.6.0

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=v21.6.0&title=21.6.0&target=release/v21.6.0

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

Author: github-actions[bot]

Cargo.lock

@@ -12,16 +12,16 @@ members = [
 ]
 
 [workspace.package]
-version = "21.5.2"
+version = "21.6.0"
 rust-version = "1.74.0"
 
 [workspace.dependencies]
-soroban-sdk = { version = "21.5.2", path = "soroban-sdk" }
-soroban-sdk-macros = { version = "21.5.2", path = "soroban-sdk-macros" }
-soroban-spec = { version = "21.5.2", path = "soroban-spec" }
-soroban-spec-rust = { version = "21.5.2", path = "soroban-spec-rust" }
-soroban-ledger-snapshot = { version = "21.5.2", path = "soroban-ledger-snapshot" }
-soroban-token-sdk = { version = "21.5.2", path = "soroban-token-sdk" }
+soroban-sdk = { version = "21.6.0", path = "soroban-sdk" }
+soroban-sdk-macros = { version = "21.6.0", path = "soroban-sdk-macros" }
+soroban-spec = { version = "21.6.0", path = "soroban-spec" }
+soroban-spec-rust = { version = "21.6.0", path = "soroban-spec-rust" }
+soroban-ledger-snapshot = { version = "21.6.0", path = "soroban-ledger-snapshot" }
+soroban-token-sdk = { version = "21.6.0", path = "soroban-token-sdk" }
 
 [workspace.dependencies.soroban-env-common]
 version = "=21.2.1"