Bump Clang CI matrix: promote 22 to primary, add 21 to secondary

- Promote Clang 22 to primary CI version (full test suite)
- Demote Clang 21 to secondary tier alongside 20 and 19 (Release.Default only)

Author: ednolan

.github/workflows/ci_tests.yml

@@ -71,7 +71,7 @@ jobs:
             }
           ],
           "clang": [
-            { "versions": ["21"],
+            { "versions": ["22"],
               "tests": [
                 {"cxxversions": ["c++26"],
                   "tests": [
@@ -90,7 +90,7 @@ jobs:
                 }
               ]
             },
-            { "versions": ["20", "19"],
+            { "versions": ["21", "20", "19"],
               "tests": [
                 { "cxxversions": ["c++26", "c++23", "c++20", "c++17"],
                   "tests": [