20.0 docs: unsupporting `gh-ost`

[shlomi-noach]

Documenting the changes in https://github.com/planetscale/vitess

Unsupporting gh-ost:

• Pointing out that gh-ost is unsupported and will be removed.
• Replacing various gh-ost examples with vitess examples.
• Other long due updates to the docs.

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[shlomi-noach]

Made final backport doc updates to 19.0, 18.0, 17.0.