Change GHC support window unit from years to {major versions, LTS, …)

See #11530

The gripe some people have with out support window is that it is expressed in years. Options proposed in the thread are:

1) x GHC major versions.
2) x GHC LTS major versions.
3) "`cabal-install` from the last non-EOL'd Ubuntu LTS (Debian stable, Fedora, whatever is your benchmark) should be able to complete `cabal install cabal-install`".
4) Keep it as it is (years).

See #11723 on whether to make our window narrower.

Start by reading the discussion in #11530 and the related issue #11723. Compare the four proposed support-window definitions—GHC major versions, GHC LTS versions, distribution support, or years—and determine which policy the project adopts; the issue is done when that choice is resolved and reflected in the relevant support-window documentation or configuration.

Written by the indexing model from the issue text.