Add note about default-auth for MySQL 8.0 clients (#2669)

[sre-bot]

cherry-pick #2669 to release-2.1

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What is changed, added or deleted? (Required)

The MySQL command line client (mysql) as well as the libmysqlclient client library have changed their default authentication plugin. See https://dev.mysql.com/doc/refman/8.0/en/upgrading-from-previous-series.html#upgrade-caching-sha2-password for more information about that change.

It's no longer possible to connect to a TiDB Server using those clients if the user account has a password set, unless the default-auth=mysql_native_password option is used.

This behavior can be seen in this example:

mysql> create user myuser identified by 'password';
Query OK, 0 rows affected (0.05 sec)

$ mysql --version
mysql  Ver 8.0.19 for osx10.15 on x86_64 (Homebrew)

$ mysql -u myuser -ppassword
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1045 (28000): Access denied for user 'myuser'@'127.0.0.1' (using password: NO)

Note that the error message says "using password: NO", even though a password is given on the command line.

The --default-auth=mysql_native_passwword option allows the connection to succeed:

$ mysql -u myuser -ppassword --default-auth=mysql_native_password -e 'select version()'
mysql: [Warning] Using a password on the command line interface can be insecure.
+------------------------------------------+
| version()                                |
+------------------------------------------+
| 5.7.25-TiDB-v4.0.0-beta.2-443-ge0ec3e3ca |
+------------------------------------------+

This patch makes several updates to pages in the documentation that deal with MySQL compatibility, setting passwords, and connecting to TiDB using MySQL clients.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

If you select two or more versions from above, to trigger the bot to cherry-pick this PR to your desired release version branch(es), you must add corresponding labels such as needs-cherry-pick-4.0, needs-cherry-pick-3.1, needs-cherry-pick-3.0, and needs-cherry-pick-2.1.

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

[yikeke]

/merge

[sre-bot]

/run-all-tests