refactor semantic conventions

[brettmc]

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)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.66%. Comparing base (3968f74) to head (af9a21b).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1659      +/-   ##
============================================
+ Coverage     68.64%   68.66%   +0.02%     
  Complexity     2854     2854              
============================================
  Files           425      425              
  Lines          8699     8699              
============================================
+ Hits           5971     5973       +2     
+ Misses         2728     2726       -2     

Flag	Coverage Δ
8.1	68.41% <ø> (+0.03%)	⬆️
8.2	68.62% <ø> (+0.03%)	⬆️
8.3	68.59% <ø> (+0.01%)	⬆️
8.4	68.59% <ø> (-0.02%)	⬇️
8.5	68.60% <ø> (ø)

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Files with missing lines	Coverage Δ
src/SemConv/Version.php	100.00% <ø> (ø)

... and 5 files with indirect coverage changes

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