internal deps: downgrade karma

It, or one its deps, was causing failures on Windows.

Windows failure symptom:
```
13 05 2022 23:04:03.964:WARN [filelist]: Pattern "D:\a\rewrite-clj\rewrite-clj\node_modules\karma-cljs-test/adapter.js" does not match any file.
13 05 2022 23:04:03.967:WARN [filelist]: Pattern "D:/a/rewrite-clj/rewrite-clj/target/cljsbuild/test/chrome-headless-none/out/goog/base.js" does not match any file.
13 05 2022 23:04:03.967:WARN [filelist]: Pattern "D:/a/rewrite-clj/rewrite-clj/target/cljsbuild/test/chrome-headless-none/out/cljs_deps.js" does not match any file.
13 05 2022 23:04:03.967:WARN [filelist]: Pattern "D:/a/rewrite-clj/rewrite-clj/target/cljsbuild/test/chrome-headless-none/compiled.js" does not match any file.
13 05 2022 23:04:03.967:WARN [filelist]: Pattern "D:/a/rewrite-clj/rewrite-clj/target/cljsbuild/test/chrome-headless-none/out/**" does not match any file.
Chrome Headless 101.0.4951.64 (Windows 10) ERROR
  You need to include some adapter that implements __karma__.start method!
```

Author: lread

package-lock.json

@@ -1,7 +1,7 @@
 {
   "name": "rewrite-clj",
   "devDependencies": {
-    "karma": "^6.3.20",
+    "karma": "^6.3.19",
     "karma-chrome-launcher": "^3.1.1",
     "karma-cljs-test": "^0.1.0",
     "karma-junit-reporter": "^2.0.0",