Derive build versions from a tracked VERSION file

[Obihoernchen]

It was hart to do tests in CI because the versioning was all over the place.
Packages built from master are stamped 3.15.2.dev<n>, a version released in April. Release tags do not live on master: 3.15.2 through 3.15.6 were tagged on branch 3.15, leaving 3.15.1 as the newest tag git describe can reach from master. Every build script bumped the last component of that.

Besides being confusing, it breaks upgrades. Both rpm and dpkg rank the released 3.15.6 above 3.15.2.dev701, so a development package will not install over a released one without forcing.

Fix

A top-level VERSION file names the release the branch is working toward (4.0.0 on master), and ./mkversion turns it into the version stamped on packages. The newest tag reachable from HEAD is kept as a floor and the higher of the two wins, so forgetting to bump VERSION after tagging the current branch cannot walk the version backwards. Staying ahead of tags on other branches is what VERSION itself is for.

situation	version
master, VERSION=4.0.0	4.0.0.dev<n>+g<hash>
branch 4.0, 5 commits past tag 4.0.0	4.0.1.dev5+g<hash>
exactly on tag 4.0.1	4.0.1
master after cutting 4.0, bumped to 4.1.0	4.1.0.dev<n>+g<hash>

Only master needs an edit, once per series, and the order matters: cut branch X.Y first, then bump VERSION on master, or the release branch inherits the wrong series. Documented in README.md.

mkversion never guesses. A missing or malformed VERSION, an unreadable git tree (build/buildarm runs buildrpm as root inside a container over a $HOME bind mount, where git refuses the repository) or an unknown argument each exit non-zero rather than emitting a plausible-looking version, and every caller propagates that.

mkversion also replaces the version block copy-pasted into seven build scripts. --tilde selects ~dev for the packages with no setup.py, as before. makesetup now echoes the version it stamped and buildrpm/builddeb capture it, so the version is computed once per package instead of the per-package VERSION file the two of them used to read; the stale checked-in confluent_common/VERSION (1.0.1, last touched in the 1.1 cycle) goes with it, leaving one file
named VERSION in the tree. The makesetup.deb wrapper in builddeb, which rewrote a tilde makesetup has not emitted in years, is dropped.

This does not change the .dev form used by confluent_server/client/common, which both rpm and dpkg rank above the matching release.

Testing

• rpmdev-vercmp and dpkg --compare-versions now rank the development build above 3.15.6
• Built and installed in the lab: alma9 upgraded from a 3.15.2.dev<n> rpm and ubuntu2404 from a
  3.15.2~dev<n> deb, both as ordinary upgrades; daemon restarted and /version reports the new version
• osdeploy tarball checked to contain confluent_osdeploy/LICENSE at the path %install uses