tablecodec: fix text type decode for old row format (#23751)

[ti-srebot]

cherry-pick #23751 to release-4.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/23772

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

git push git@github.com:ti-srebot/tidb.git pr/23772:release-4.0-fb2df7675c64

---

What problem does this PR solve?

Issue Number: close #23708

Problem Summary:
Didn't set collation. It's the patch for #23045

What is changed and how it works?

Set collation for unflaten.

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Manual test
  Star a tiup cluster, and do the operations as the issue says.
  admin check table, no error.

Release note

• fix text type decode for old row format

[ti-srebot]

/run-all-tests

[ti-srebot]

@wjhuang2016 you're already a collaborator in bot's repo.

[zimulala]

/LGTM

[tangenta]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• tangenta
• zimulala

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[wjhuang2016]

/merge

[wjhuang2016]

/merge

[zhouqiang-cl]

/merge

[XuHuaiyu]

/merge cancel

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: da4005b

[ti-chi-bot]

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