Disable Java 21 testing

`lein eastwood` fails on Java 21 with:

```
Caused by: clojure.lang.Compiler$CompilerException: Syntax error macroexpanding at (/home/runner/work/clj-http-client/clj-http-client/project.clj:1:1).
37
```

Author: bastelfreak

.github/workflows/pr-testing.yaml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: ['17', '21']
+        version: ['17',]
     runs-on: ubuntu-latest
     steps:
       - name: checkout repo