Add reminder to update MSRV in GH Action's CI suite. (#709)

.github/workflows/ci.yml

@@ -154,6 +154,9 @@ jobs:
         # IMPORTANT: When updating the versions here, make sure you also update
         #            the `fails-on-too-old-rustc` job and its error message to
         #            to account for the raised minimum required version.
+        #
+        #            Also make sure to update the MSRV in the cargo-semver-checks-action CI:
+        #            https://github.com/obi1kenobi/cargo-semver-checks-action/blob/main/.github/workflows/test-action.yml#L18
         toolchain: ["1.74", "1.75", "stable", "beta"]
         experimental: [false]
         include: