Update old ecosystem checks for formatter for clarity (#8285)

Changes the title and adds some notes re the old formatter ecosystem
checks in light of #8223

Does not remove it as I'm not sure where else we test for instabilities.

scripts/formatter_ecosystem_checks.sh

@@ -1,4 +1,9 @@
 #!/usr/bin/env bash
+# **NOTE**
+# This script is being replaced by the ruff-ecosystem package which is no
+# longer focused on black-compatibility but on changes in formatting between
+# ruff versions. ruff-ecosystem does not support instability checks yet.
+#
 # Check black compatibility and check for formatter instabilities and other
 # errors.
 #