Stop calling fixed format cache experimental (#19967)

It has been around for some time with no issues so far. We can stop
calling it experimental so that more people try it before we make it
default.

Author: ilevkivskyi

mypy/main.py

@@ -1066,11 +1066,7 @@ def add_invertible_flag(
     incremental_group.add_argument(
         "--fixed-format-cache",
         action="store_true",
-        help=(
-            "Use experimental fast and compact fixed format cache"
-            if compilation_status == "yes"
-            else argparse.SUPPRESS
-        ),
+        help="Use new fast and compact fixed format cache",
     )
     incremental_group.add_argument(
         "--skip-version-check",