Life times / support windows for `cabal-install`

Triggered by #11111: Because of upstream change of root keys for hackage, `cabal` < 3.10.2 stopped working from one day to the other. This is not good.
Confronted with the breakage of old versions of `cabal`, one wonders what one should expect, i.e., how long should an old `cabal` remain functional (for at least the basic stuff like `cabal update` and `cabal install cabal-install`), and when can it be dropped for good?

E.g. the latest Ubuntu LTS (24.04) ships 3.8, so it is to stay until April 2029. With 3.8 released in August 22, this would be a lifetime of almost 7 years.
Maybe one can expect most Ubuntu users to switch to the newer LTS after some good time (say a year), this would mean that `cabal-3.8` need not be functional after April 2027, giving it a life time of short of 5 years.

Would a support window of 5 years for `cabal-install` be reasonable? (This would only concern essential functionality, not the backporting of any bug fix.)

I think it would be good to have a document stating the intended life times of `cabal`.
`cabal` being the frontend to `hackage`, such support pledges would also involve the Hackage crew. @gbaz @jmct

Original comment:
- https://github.com/haskell/cabal/issues/11111#issuecomment-3135423652