refactor: rework mime type comparison and clean code rest.py: accept application/* as match for application/json in non /binary_context rest path. allow defining default mime type to return when file/message is missing mime type. Make it a class variable to it can be changed from text/plain to text/markdown or whatever. extract code from determine_output_format() to create create_valid_content_types() method which returns a list of matching mime types for a given type/subtype. Eliminate mostly duplicate return statements by introducing a variable to specify valid mime types in error message. rest_common.py: Fix error messages that now return application/* as valid mime type. CHANGES.txt upgrading.txt rest.txt: top level notes and corrections. Also correct rst syntax on earlier change.

# https://issues.roundup-tracker.org/issue2551278
# datetime.utcnow deprecated
try:
    from datetime import UTC, datetime

    def utcnow():
        return datetime.now(UTC)

except ImportError:
    from datetime import datetime

    def utcnow():
        return datetime.utcnow()