session: fix bug user privileges change after upgrade from 4.0.11 to 5.0 (#23403)

[ti-srebot]

cherry-pick #23403 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/23527

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/23527:release-5.0-5a4d89416bc0

---

What problem does this PR solve?

Issue Number: close #23387

Problem Summary:

#21856 introduce the problem, it adds Repl_slave_priv to all existing users while it should just do that for the root user.

What is changed and how it works?

What's Changed:

Just assign the newly introduced privileges to the root user, rather than all existing users.

How it Works:

Make the behavior more acceptable when upgrading.

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Side effects

This just fix the code, if a user upgrades his cluster and meets the bug, he have to fix the data manually.

Release note

• Fix a bug when upgrading from 4.X to 5.0, the existing users obtain REPLICATION CLIENT and REPLICATION privileges unexpectedly.

[ti-srebot]

/run-all-tests

[ti-srebot]

No release note, Please follow https://github.com/pingcap/community/blob/master/contributors/release-note-checker.md

[ti-srebot]

@tiancaiamao you're already a collaborator in bot's repo.

[zimulala]

/LTGM

[zimulala]

/cc @xhebox

[tiancaiamao]

/rebuild

[djshow832]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• djshow832
• zimulala

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[zimulala]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 387541c

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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