Introduce support for VERSION file

This change set introduces support for the `VERSION` file used by other
GitLab projects (like Gitaly). It contains the SSoT for the version that
is built - especially for releases.
Using the version from this file is the fallback for when the build is
NOT happening in a git source tree - this is the case for official
GitLab KAS releases via CNG and Omnibus.

When building in a git source tree the `VERSION` file is NOT used, but
the closest git tag with a commit reference suffix is used that is
produced by `git describe`.

This change set also implies that the `make` interface changed, because
it no longer supports providing the `GIT_COMMIT` and `GIT_TAG` arguments
that would in turn have been used for the release versioning.
However, this is still a non-breaking change because the argument will
just not be respected.

Preconditions for this change set to work properly:

- [ ] release-tools needs to support the `VERSION` file for KAS.
- [ ] (optional) change CNG to no longer provide the `GIT_TAG` and
  `GIT_COMMIT` make arguments.
- [ ] (optional) change Omnibus to no longer provide the `GIT_TAG` and
  `GIT_COMMIT` make arguments.

Changelog: other