Add RenameBranch support

[vlsokol]

Add support for rename a branch: https://docs.github.com/en/rest/reference/repos#rename-a-branch
Fixes: #2118

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[vlsokol]

@googlebot I signed it!

[codecov]

Codecov Report

Merging #2119 (c9e1ab6) into master (f664094) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2119   +/-   ##
=======================================
  Coverage   97.78%   97.78%           
=======================================
  Files         112      112           
  Lines        9956     9969   +13     
=======================================
+ Hits         9735     9748   +13     
  Misses        154      154           
  Partials       67       67           

Impacted Files	Coverage Δ
github/repos.go	98.67% <100.00%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f664094...c9e1ab6. Read the comment docs.

[gmlewis]

Thank you, @vlsokol !
Let me know what you think about simplifying the API endpoint:

[gmlewis]

This looks great, @vlsokol ... thank you!
LGTM.

Awaiting second LGTM (from any repo contributor) before merging.

[gmlewis]

Thank you, @AGMETEOR !
Merging.