Made approximateBacklogCounter available via legacy API

[Shivs11]

What changed?

• approximateBacklogCounter to be available via legacy DescribeTaskQueue API
• Legacy BacklogCountHint is removed. I propose that the new approximateBacklogCounter is a better estimate for task backlog and has been replaced.
• Functional test checking if the in-memory backlog counter is updated as expected
• Updated unit tests which were dealing with the legacy API

Why?

• It is better to emit the more accurate backlog counter, which in this case, would be approximateBacklogCounter.

How did you test it?

• Current CI/CD tests
• Functional test added

Potential risks

None since this is merging into my own feature branch.

Is hotfix candidate?

No