Align with how the spec advises SemConv should be organised, and break incubating (stable + unstable) semconvs out into their own directory.
I played around with generating values as enums, but I could only get this to work if the output was in the same file as the attributes (which would not work well with composer autoloading), so I've used <attribute>_VALUE_<name>, similar to what we have now except all in the same file as the attributes.

Discussion:

• From here on, we will not regenerate our existing *Attributes and *AttributeValues files, but try to memorialize them to preserve BC (to discuss: marking these as deprecated now will make a lot of pipelines unhappy, should we wait?)
• I haven't included any deprecated entries for both stable/incubating. I don't think weaver has a way to filter deprecations from a point-in-time, so it's all-or nothing. If there are deprecations of stable semconvs in the future, we'll need to deal with that (for reference, Java excludes deprecations from their stable SemConv generation: https://github.com/open-telemetry/semantic-conventions-java/blob/release/v1.34.0/buildscripts/templates/registry/java/weaver.yaml#L24)