Reduce new test concurrency to five (#94552)
### What?
Reduce the `test-new-tests-dev` and `test-new-tests-start` workflow
matrices from ten groups to five.
### Why?
Restore these flake-detection jobs to five concurrent shards.
### How?
Change each matrix group list from `1/10` through `10/10` to `1/5`
through `5/5`.
### Verification
- `pnpm prettier --with-node-modules --ignore-path .prettierignore
--check .github/workflows/build_and_test.yml`
- `git diff --cached --check`
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