Constrained Tip-Tracking: use cgroups v2

cgexec is for cgroups v1 and systemd-run is for cgroups v2

Enhance the test using systemd-run.

See note [here](https://github.com/erigontech/erigon/pull/17497#discussion_r2471566146).

Current cgroup [definition](https://github.com/erigontech/erigon-qa/blob/1daf63bf0cb4a659a762710b5b48e0bd2ffb969d/config_management/test_runner_node.yml#L191) and [usage](https://github.com/erigontech/erigon/blob/4134d3f8ad6fdc1278fb3aa61d616481665740d5/.github/workflows/qa-constrained-tip-tracking.yml#L97).

Equivalent command that uses systemd-run:

```bash
systemd-run --scope \
--uid=runner_user \
--gid=runner_user \
-p MemoryMax=32G \
-p MemorySwapMax=infinity \
-- bash -lc 'python3 <params>'
```