planner: Revert tidb_allow_mpp modification for downgrade compatibility and add warnings for enforce mpp. (#25302)

[ti-srebot]

cherry-pick #25302 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/25358

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

git push [email protected]:ti-srebot/tidb.git pr/25358:release-5.0-1f0245a82142

---

What problem does this PR solve?

Issue Number: close pingcap/tiflash#1905

Problem Summary: #24516 will let TiDB can't downgrade in some cases.

What is changed and how it works?

Proposal: Enofrce MPP

What's Changed: This PR reverted #24516, introduce a new variable tidb_enforce_mpp and some warning.

How it Works:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• 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

• Performance regression
  • Consumes more CPU
  • Consumes more MEM
• Breaking backward compatibility

Release note

• Revert

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[LittleFall]

seems no need to cherry-pick to 5.0? @hanfei1991

[ti-chi-bot]

@ti-srebot: PR needs rebase.

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[LittleFall]

has been cherry-picked correctly in #26382