Track deprecated Beta features w/ earliest date of removal

[ghost]

Changes

We don't currently have a centralized place to list the
features that Pipelines has deprecated.

This commit adds a deprecations doc which shows the feature
being deprecated, a link to the release where its deprecation
was announced, and the earliest date by which the feature
will be removed entirely in relation to the announcement.

At the moment I've only listed Beta features that have been
announced as deprecated. I haven't included things which are
still in Alpha or were announced as deprecated prior to
v0.11.0-rc1, since the policy for alpha code was simply that
we'd give 1 release of forewarning.

Submitter Checklist

• Includes docs (if user facing)
• Commit messages follow commit message best practices

Release Notes

A list of feature deprecations along with the earliest date of their removal has been added to Pipelines' docs.

[bobcatfish]

/approve
/lgtm

Didn't seem worth blocking on but would it makes sense for the deprecation doc to link to https://github.com/tektoncd/pipeline/blob/master/api_compatibility_policy.md and maybe vice versa?

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[ghost]

Didn't seem worth blocking on but would it makes sense for the deprecation doc to link to https://github.com/tektoncd/pipeline/blob/master/api_compatibility_policy.md and maybe vice versa?

Good call. We already do link to the policy from this doc (the API Compatibility Policy heading in the table links to it), but we can also add a link from the policy doc to this one.