CI(pg-clients): unify workflow with build-and-test (#8160)

## Problem

`pg-clients` workflow looks different from the main `build-and-test`
workflow for historical reasons (it was my very first task at Neon, and 
back then I wasn't really familiar with the rest of the CI pipelines).
This PR unifies `pg-clients` workflow with `build-and-test`

## Summary of changes
- Rename `pg_clients.yml` to `pg-clients.yml`
- Run the workflow on changes in relevant files
- Create Allure report for tests
- Send slack notifications to `#on-call-qa-staging-stream` channel
(instead of `#on-call-staging-stream`)
- Update Client libraries once we're here

test_runner/pg_clients/python/pg8000/requirements.txt

@@ -1,2 +1,2 @@
-pg8000==1.30.5
+pg8000==1.31.2
 scramp>=1.4.3