Clarify GHC support window

Author: ffaf1

CONTRIBUTING.md

@@ -267,18 +267,25 @@ you push a fix of a whitespace violation, please do so in a _separate commit_. F
   support window, except Template Haskell, which would cause
   bootstrapping problems in the GHC compilation process.
 
-* Our GHC support window is five years for the Cabal library and three
-  years for cabal-install: that is, the Cabal library must be
-  buildable out-of-the-box with the dependencies that shipped with GHC
-  for at least five years.  GitHub Actions checks this, so most
-  developers submit a PR to see if their code works on all these
-  versions of GHC.  `cabal-install` must also be buildable on all
-  supported GHCs, although it does not have to be buildable
-  out-of-the-box. Instead, the `cabal-install/bootstrap.sh` script
-  must be able to download and install all of the dependencies (this
-  is also checked by CI). Also, self-upgrade to the latest version
-  (i.e. `cabal install cabal-install`) must work with all versions of
-  `cabal-install` released during the last three years.
+* Our GHC support window is five years for both the Cabal library and
+  `cabal-install`.  That is:
+
+  * for at least five years, the Cabal library must be buildable
+    out-of-the-box with the dependencies that shipped with GHC;
+  * for at least five years, `cabal-install` should be buildable with
+    `cabal-install/bootstrap.sh` script;
+  * `cabal-install` should work with the most recent version of any GHCs
+    major version released in the past five years;
+
+  Self-upgrade to the latest version (i.e. `cabal install cabal-install`)
+  must work with all versions of `cabal-install` released during the last
+  three years.
+
+  GitHub Actions checks GHC-support, so most developers submit a PR
+  to see if their code works on all these versions of GHC.
+
+  Cabal maintainers try to support GHC versions older than five years
+  on best-effort basis.
 
 * `Cabal` has its own Prelude, in `Distribution.Compat.Prelude`,
   that provides a compatibility layer and exports some commonly