util/charset: Add utf8mb4 as alias for utf8

[morgo]

Supports applications which directly change to utf8mb4 charset.
Safe for TiDB to treat both as the same.
Fixes #7267

What have you changed? (mandatory)

Added an alias for utf8mb4 as a character set. It is treated the same as utf8.

What is the type of the changes? (mandatory)

Bug fix/compatibility improvement.

How has this PR been tested? (mandatory)

Manual testing:

mysql> SELECT CONVERT( CONVERT( 'xxx' USING ascii ) USING utf8mb4 ) AS xxx;
+------+
| xxx  |
+------+
| xxx  |
+------+
1 row in set (0.00 sec)

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

No

Does this PR affect tidb-ansible update? (mandatory)

No

Does this PR need to be added to the release notes? (mandatory)

Release note: Compatibility fix: utf8mb4 has been added as an alias for utf8.

Refer to a related PR or issue link (optional)

Fixes #7267

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[shenli]

@morgo Please add a test case for this PR.

[shenli]

/run-all-tests

[lysu]

/run-unit-test

[shenli]

/run-all-tests

[shenli]

LGTM

[zz-jason]

LGTM