system variables: document tidb_opt_index_join_build_v2 compat

[AilinKid]

What is changed, added or deleted? (Required)

Document the compatibility behavior of tidb_opt_index_join_build_v2.

• Add the system variable entry in system-variables.md.
• Clarify that TiDB always uses index join build v2 now.
• Clarify that setting this variable to OFF returns an error and cannot switch back to v1.
• Clarify that historical values left in mysql.global_variables after upgrade are still treated as ON.
• Add the corresponding entry to system-variable-reference.md.

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

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

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

• This PR is translated from:
• Other reference link(s):
  • planner, sessionctx: remove index join build v1 tidb#66871

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[qiancai]

@AilinKid Please invite a tech reviewer to review this PR. Thanks.