AppVeyor: Don't test GHC <8.4

See the discussion starting at
haskell/text#404 (comment)
for more context.

appveyor.yml

@@ -10,10 +10,9 @@ environment:
     - GHCVER: "8.8.4.1"
     - GHCVER: "8.6.5"
     - GHCVER: "8.4.4"
-    - GHCVER: "8.2.2"
-    - GHCVER: "8.0.2.2"
-    - GHCVER: "7.10.3.2"
-    - GHCVER: "7.8.4.1"
+    # We deliberately do not test any versions older than GHC 9.4, as GHC 8.2
+    # is unable to build text-2.0 on Windows. See the discussion at
+    # https://github.com/haskell/text/pull/404.
 
 cache:
  - "C:\\SR"