Upgrading tests to use timezone.utc rather than deprecated utcnow()

[SOORAJTS2001]

fix: modify datetime.datetime.utcnow() to datetime.datetime.now(datetime.timezone.utc) inside tests since utcnow() is deprecated from 3.12 onwards fix: change code inside defining-documents.rst and signals.rst to accommodate datetime.datetime.now(datetime.timezone.utc)

This is a follow up for the Issue #2883

@bagerard @rozza can you please take a look into it