chore(repo): Increase number of concurrently running nx tasks (#19443)

After moving from lerna to nx, I noticed we run fewer tasks in
parrallel. This PR sets the limit to ~10~ 5. previously, we used the
[default
value](https://nx.dev/docs/guides/tasks--caching/run-tasks-in-parallel)
3.

If reviewers prefer a different number than ~10~ 5, I'm happy to change
it. On my local build this led to good results but mileage may vary.

UPDATE: I reduced the number from 10 to 5 because 10 concurrent tasks
produced random test fails on CI. After a bit or research, this most
likely relates to memory pressure or CPU starvation, due to many
parallel running threads. Vitest and Playwright run tests in multiple
threads, so if we multiply this with more concurrently running tests,
this produceds a lot of threads, especially in resource-constrained CI
runners. We could experiment further with a higher threshold but for now
I'd keep 5. If we run into inexplicable test fails, this is the PR we
need to revert.

Closes #19444 (added automatically)

Author: Lms24

nx.json

@@ -64,5 +64,6 @@
   "cacheDirectory": ".nxcache",
   "tui": {
     "autoExit": true
-  }
+  },
+  "parallel": 5
 }