feat(spanner): [Spanner] A new field snapshot_timestamp is added to message .google.spanner.v1.CommitResponse (#8452)

* feat(spanner): A new field `snapshot_timestamp` is added to message `.google.spanner.v1.CommitResponse`
docs(spanner): A comment for field `commit_stats` in message `.google.spanner.v1.CommitResponse` is changed
docs(spanner): A comment for field `precommit_token` in message `.google.spanner.v1.CommitResponse` is changed
docs(spanner): A comment for message `.google.spanner.v1.TransactionOptions` is changed
docs(spanner): A comment for enum value `READ_LOCK_MODE_UNSPECIFIED` in enum `ReadLockMode` is changed
docs(spanner): A comment for enum value `PESSIMISTIC` in enum `ReadLockMode` is changed
docs(spanner): A comment for enum value `OPTIMISTIC` in enum `ReadLockMode` is changed
docs(spanner): A comment for field `multiplexed_session_previous_transaction_id` in message `.google.spanner.v1.TransactionOptions` is changed
docs(spanner): A comment for field `exclude_txn_from_change_streams` in message `.google.spanner.v1.TransactionOptions` is changed
docs(spanner): A comment for message `.google.spanner.v1.MultiplexedSessionPrecommitToken` is changed

PiperOrigin-RevId: 784456709

Source-Link: googleapis/googleapis@62babf2

Source-Link: googleapis/googleapis-gen@c1d5fdd
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* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]