ci: update actions/upload-artifact to v4 with merge

[joshuarli]

Uses artifacts/merge to merge together multiple artifacts from different jobs into the single one named github.sha that craft expects.

Summarily, upload-artifact v3 is deprecated but v4 doesn't support mutating an artifact with the name name by uploading different filepaths to the same artifact. Because we need a single artifact "github.sha", we have to use actions/merge to create it. Alternatively craft could be modified but this is the easiest way forward and I like the idea of a unified artifact, it makes craft simpler.

ref: getsentry/craft#552

This has already been done for numerous other repos.

[joshuarli]

the release artifact built here is actually an improvement since it contains both wheels (current github releases only has sentry_jsonnet wheel):

josh@CTKP7QRX4L.local ~/Downloads  $ unzip -l b6282787f1ecde332724553d90b2d1f132f33f83.zip                                                                0
Archive:  b6282787f1ecde332724553d90b2d1f132f33f83.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
     3521  09-23-2024 17:45   sentry_jsonish-0.0.2-py3-none-any.whl
     3054  09-23-2024 17:45   sentry_jsonnet-0.0.4-py3-none-any.whl
---------                     -------
     6575                     2 files