Deprecate tap pinning (and remove odisabled)

[MikeMcQuaid]

Tap pinning is not used by Homebrew or the Homebrew maintainers so issues relating to pinned taps (#5418, #5796) don’t get fixed. Tap pinning does not work consistently or reliably and is conceptually confusing to encourage users to use fully-scoped user/tap/formula naming or avoid shadowing core tap formulae' names instead.

While we are handling deprecations: remove all odisabled.

This will not be merged until the next minor release is imminent.

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[slhck]

fully-scoped user/tap/formula naming or avoid shadowing core tap formulae' names.

To a novice user, that wording may be confusing. When the user calls tap-pin, it can be assumed that they explicitly want to override the default. Perhaps one could immediately print:

brew tap-pin foo/bar (where foo/bar is whatever the user wrote) is deprecated. If you want to install formulae from foo/bar, always use the full formula name (foo/bar/<formula>).

Also, the "is shadowed by" message that is printed when the user does something with a tap formula that shadows a core formula … maybe it could mention that this behavior is going away?

[MikeMcQuaid]

@slhck Good points, I've updated some of the messaging.