Online DDL: removal of gh-ost and pt-osc strategies

[shlomi-noach]

Description

This PR completely removes any gh-ost and pt-online-schema-change support, functionality, and flags from Vitess Online DDL.

This means all of the following yields errors:

$ vtctldclient ApplySchema --ddl-strategy="gh-ost" ...
$ vtctldclient ApplySchema --ddl-strategy="pt-osc" ...
$ vttablet --gh-ost-path=...
$ vttablet --pt-osc-path=...

The remaining strategies are:

• vitess (aka online, both are allowed)
• mysql (not Online, but still managed)
• direct (unmanaged)

Requires documentation updates! website/docs issue TBD.

Related Issue(s)

• Tracking: deprecation of gh-ost and pt-osc Online DDL strategies #15692
• Online DDL: unsupporting gh-ost DDL strategy #15693
• 20.0 docs: unsupporting gh-ost website#1725

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.80%. Comparing base (1d57f39) to head (4f4684f).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17626      +/-   ##
==========================================
+ Coverage   67.68%   67.80%   +0.12%     
==========================================
  Files        1586     1585       -1     
  Lines      255413   254971     -442     
==========================================
+ Hits       172883   172893      +10     
+ Misses      82530    82078     -452     

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