remove enum defaults, and some other cleanup

[cosmicexplorer]

@illicitonion left some very useful comments on #7226, one specifically asking why enums have defaults: #7226 (review). This led me to immediately realize that all of the complexity of the argument manipulation with *args, **kwargs in the .create() classmethod mentioned in #7226 (comment) was due to trying to support default values "hygienically", and the only thing that really needed a default value was Platform:

pants/src/python/pants/backend/native/config/environment.py

Lines 17 to 19 in 1ece461

	class Platform(enum('normalized_os_name', all_normalized_os_names())):
	default_value = get_normalized_os_name()

which really should only expose the current os name as the default value. The default value part was also maybe useful for registering enums as options directly, but we've now laid out how we want to do that in #7233. This ticket covers:

• Remove any concept of enums having a default value, which should also allow us to turn the factory method into def create(value): without any complicated argument specs.
• Expose an @memoized_classproperty as Platform.current and convert all usages of its .create() method with no args (all of them) to use that instead.
• Avoid directly accessing the enum field with .value as per Canonicalize enum pattern matching for execution strategy, platform, and elsewhere #7226 (comment). Also, deprecate the field_name argument and remove all such usages in the pants repo.
• See if the getattr calls are all necessary and/or if the logic can be documented more as per Canonicalize enum pattern matching for execution strategy, platform, and elsewhere #7226 (comment).