This replaces gpkg-to-pgdump.yml with the following changes:

- opendrr-boundaries.sql in the git repo is used directly
- opendrr-boundaries.dump is no longer built in the workflow
- opendrr-boundaries.sql is split into 2GB-chunks before uploading as
  release assets (#34)
- No longer uploads to Amazon S3 and Backblaze B2 buckets
- GeoPackages (*.gpkg) are no longer uploaded by this workflow;
  they will be handled by a new upload-geopackages.yml instead

Fixes #34
See also #36

Downloading a large GitHub artifact can sometimes be very slow,
with a 3GB artifact taking over 45 minutes, so keeping a copy on a
Backblaze B2 bucket allows faster download if necessary.

This uploads *.gpkg files as release assets as the old
gpkg-to-pgdump.yml workflow used to do.

As softprops/action-gh-release@v1 currently fails to upload all 6.0 GiB
of GeoPackages at once, xresloader/upload-to-github-release@v1 is now
used for the task.

See #36

This workflow prevents the checked-out Git LFS files in the
workflow cache from getting stale and removed by accessing the cache
every Sunday at 00:38 or 01:38 Pacific Time.

Upstream has updated from Node 12 to Node 16 in these GitHub Actions.

Note: actions/cache v3.0.0 has trouble restoring cache over
2 GiB in size:

    Warning: The value of "length" is out of range.
    It must be >= 0 && <= 2147483647. Received 2419791709

so the upgrade to actions/cache@v3 will have to wait until the
resolution of Issue actions/cache#773 in PR actions/cache#775.

into new file upload-sqldump-and-geopackages.yml.

The 'prevent-race-condition' concurrency is moved from job level
to workflow level in order to prevent workflow cancellation:

    Canceling since a higher priority waiting request
    for 'prevent-race-condition' exists

Proper workflow_dispatch is also implemented in case there is a need to
manually (re)upload the release assets.

Together with the previous commits, the refactoring of the previous
gpkg-to-pgdump.yml GitHub Actions workflow is now complete.

Fixes #36