fix[stac_fastapi.core]: await request.body() coroutine

[StijnCaerts]

Related Issue(s):
N/A

Description:
Fixes an unawaited coroutine:

stac_fastapi/core/core.py:762: RuntimeWarning: coroutine 'Request.body' was never awaited
  if request.method == "POST" and request.body():

request.body() always had a bool value of True as it is the coroutine, not the result.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable
• Changes are added to the changelog

[jonhealy1]

Thanks @StijnCaerts