chore: update pinned rust to v 1.75

Author: JosephLenton

.github/workflows/ci.yml

@@ -61,7 +61,7 @@ jobs:
         include:
           - build: pinned
             os: ubuntu-20.04
-            rust: 1.72
+            rust: 1.75
             # Fails on pinned version because the output changed,
             # so we're excluding it, but it's still tested on stable and nightly.
             EXCLUDE_UI_TESTS: "pattern_mismatched_types,newtype,matches_unknown_field"