doc(release): update release docs with BCR instructions

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request updates the release documentation in release/README.md by removing outdated steps, updating GitHub release instructions, and adding a detailed guide for updating the google_cloud_cpp Bazel Central Registry (BCR) module. The review feedback identifies several issues in the new BCR instructions, including incorrect version variable prefixes, a typo, a hardcoded version in a heading, and a missing variable assignment that would break the checksum replacement command.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (95b11e1) to head (c55549c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16329      +/-   ##
==========================================
- Coverage   92.24%   92.24%   -0.01%     
==========================================
  Files        2227     2227              
  Lines      209169   209169              
==========================================
- Hits       192943   192938       -5     
- Misses      16226    16231       +5     

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[colinmoy]

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