google/cloud-spanner v2.6.0

<<<<<<< Updated upstream

� BREAKING CHANGES

• Promote Logging to V2 (#9007)

Stashed changes

Features

• [Spanner] include cache updates and routing hint into BeginTransaction and Commit request/response respectively (#8962) (28d8a69)

Documentation

• A comment in message .google.spanner.v1.TransactionOptions.ReadWrite.ReadLockMode is changed (28d8a69)