Move .codecov.yml (temporalio#7587)

## What changed?
<!-- Describe what has changed in this PR -->

Moved `.codecov.yml` into `.github/`

## Why?
<!-- Tell your future self why have you made these changes -->

Reduce number of top-level files (clutter).

## How did you test it?
<!-- How have you verified this change? Tested locally? Added a unit
test? Checked in staging env? -->

This branch still reports code coverage:
https://app.codecov.io/gh/temporalio/temporal/tree/stephanos%2Ftemporal%3Amove-codecov

## Potential risks
<!-- Assuming the worst case, what can be broken when deploying this
change to production? -->

From codecov FAQ:

> **Can I name the file .codecov.yml?**
Yes, you can name the file codecov.yml or .codecov.yml. However, the
file must still be located in the repository root, dev/, or .github/
directories

## Documentation
<!-- Have you made sure this change doesn't falsify anything currently
stated in `docs/`? If significant
new behavior is added, have you described that in `docs/`? -->

## Is hotfix candidate?
<!-- Is this PR a hotfix candidate or does it require a notification to
be sent to the broader community? (Yes/No) -->

Author: stephanos