Improve & simplify valgrind testing

[onurctirtir]

• Reduce the dependency on the other constructs of this repo, e.g., creation & deallocation of Azure VMs via the existing tooling doesn't seem so much stable. So, introduce a simple docker image that can allow us running multiple valgrind tests in a single vm in parallel and anywhere. Optionally, create_cluster.sh and its friends can still be used to create an Azure VM but they are not a must now. See the changes in README.md for more details.
• Get rid of valgrind code from fab infra because using this infra for valgrind testing was both unnecessary and this was heavily complicating the things. Instead, a simpler infra that we can also use in our local machines seems much more useful and easier to maintain and understand.
• Also remove fabfile/pg_report_vg_query.patch since the Postgres patch that we depend on to report the query string that causes the memory error has been merged into Postgres as of PG16.
• Automatically collect stack traces from valgrind core files that are generated in case of a process crash.

Note to reviewer - although the PR touches nine files, this PR mainly introduces following three files and updates the README on the usage of valgrind testing infra. The other 5 files are only modified to remove the old infra.

• valgrind/Docker/Dockerfile
  The dockerfile to build a simple docker image that can be used to run valgrind tests.
  This has nothing but Postgres & Citus built with the flags that are needed to properly run valgrind tests and valgrind itself.
• valgrind/Docker/run_valgrind_test.sh
  The script that we run within the container to run the valgrind test for the specified test schedule.
  We could set the entrypoint of the container to this script but we keep it as a separate script for simplicity.
• run.sh
  The host side script that we use to build the container and run the valgrind tests through the container.

I strongly recommend reading the relevant section of the README from this PR to better understand the changes.