Tablet throttler: throttler-config-via-topo defaults 'true', deprecation message for old flags (alternate)

[shlomi-noach]

Description

Alternate PR to #13130, seeing that #13130 has diverged a bit and we're experimenting with a few options.

• v16 introduced a new vttablet flag: --throttler-config-via-topo, see the docs: https://vitess.io/docs/16.0/reference/features/tablet-throttler/
• In v16 this flag defaults false, and the old per-tablet --enable-lag-throttler configuration is still supported.
• For v17, the target of this PR, this PR sets the default for --throttler-config-via-topo to true. The old configuration is still supported but if used there's a deprecation warning.
• In v18, the old configuration & logic will be compeletely removed and it will be assumed that --throttler-config-via-topo is always true whether specified or not. The flag will issue a deprecation message.
• In v19 we will remove the flag --throttler-config-via-topo.

We remove all references to --enable-lag-throttler in Vitess's own endtoend tests, and use the dynamic throttler config everywhere.

Related Issue(s)

Follow up to:

• Dynamic tablet throttler config: enable/disable, set metrics query/threshold #11604
• Throttler: Store Config in Global Keyspace Topo Record #12520

Documentation PR to follow.

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• If this is a change that users need to know about, please apply the release notes (needs details) label so that merging is blocked unless the summary release notes document is included.
• If a test is added or modified, there should be a documentation on top of the test to explain what the expected behavior is what the test does.

If a new flag is being introduced:

• Is it really necessary to add this flag?
• Flag names should be clear and intuitive (as far as possible)
• Help text should be descriptive.
• Flag names should use dashes (-) as word separators rather than underscores (_).

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow should be required, the maintainer team should be notified.

Bug fixes

• There should be at least one unit or end-to-end test.
• The Pull Request description should include a link to an issue that describes the bug.

Non-trivial changes

• There should be some code comments as to why things are implemented the way they are.

New/Existing features

• Should be documented, either by modifying the existing documentation or creating new documentation.
• New features should have a link to a feature request issue or an RFC that documents the use cases, corner cases and test cases.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• vtctl command output order should be stable and awk-able.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from VTop, if used there.