[Direct PR][release-21.0] Add RPC changes segment in the summary doc

[GuptaManan100]

Description

We think it's a good idea to also include a segment that just talks about the changes made to RPC calls in the release. This will help users see at a glance what RPCs have changed across different versions.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
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Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[rohit-nayak-ps]

Here are the changes generated by the script given below

https://gist.github.com/rohit-nayak-ps/ff70493906f84094e66f92262f9be842

git checkout v20.0.0
cp -r proto /tmp/proto20
git checkout v21.0.0-rc2
cp -r proto /tmp/proto21
diff /tmp/proto2* > proto.diff