v5.2.2: Update TLS docs (#6323)

[ti-chi-bot]

This is an automated cherry-pick of #6323

What is changed, added or deleted? (Required)

Update TLS docs with:

• TiDB 5.2.x additions
• Remove the use of mysql_ssl_rsa_setup as this is replaced with AutoTLS
• Add SSL Modes available in MySQL 8.0 client

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

• master (the latest development version)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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

Closes #6387

Including this:

• Add docs for auto-tls setting #6125
• server: Add status vars for NotAfter/NotBefore of TLS certs tidb#26132
• util: Automatically create TLS certificates tidb#24141

Not including this as it doesn't apply to v5.2:

• config,util: Add setting for TLS version tidb#26937

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: a24f017