*: move config file options prepared-plan-cache.* to sysvars

[CbcWestwolf]

What problem does this PR solve?

Issue Number: ref #33769, close #30168

Previous closing PR: #33836

Problem Summary:

The options prepared-plan-cache.* have historically been config options. But based on requirements from cloud & PM they should instead be sysvars.

What is changed and how it works?

Remove them from the config list and add them to global sysvars.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

The TiDB configuration file options `prepared-plan-cache-*` has been replaced by the system variables `tidb_enable_prepared_plan_cache`, `tidb_prepared_plan_cache_size`, and `tidb_prepared_plan_cache_memory_guard_ratio`. This change makes it easier to modify this setting on a cluster wide basis.

[ti-chi-bot]

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[ti-chi-bot]

@CbcWestwolf: PR needs rebase.

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