See https://github.com/micropython/micropython/issues/12127 for details.

Previously at the point when a release is made, we update mpconfig.h
and set a git tag. i.e. the version increments at the release.

Now the version increments immediately after the release. The workflow is:
1. Final commit in the cycle updates mpconfig.h to set (X, Y, 0, 0) (i.e.
   clear the pre-release state).
2. This commit is tagged "vX.Y.0".
3. First commit for the new cycle updates mpconfig.h to set (X, Y+1, 0, 1)
   (i.e. increment the minor version, set the pre-release state).
4. This commit is tagged "vX.Y+1.0-preview".

The idea is that a nightly build is actually a "preview" of the _next_
release. i.e. any documentation describing the current release may not
actually match the nightly build. So we use "preview" as our semver
pre-release identifier.

Changes in this commit:
 - Add MICROPY_VERSION_PRERELEASE to mpconfig.h to allow indicating that
   this is not a release version.
 - Remove unused MICROPY_VERSION integer.
 - Append "-preview" to MICROPY_VERSION_STRING when the pre-release state
   is set.
 - Update py/makeversionhdr.py to no longer generate MICROPY_GIT_HASH.
 - Remove the one place MICROPY_GIT_HASH was used (it can use
   MICROPY_GIT_TAG instead).
 - Update py/makeversionhdr.py to also understand
   MICROPY_VERSION_PRERELEASE in mpconfig.h.
 - Update py/makeversionhdr.py to convert the git-describe output into
   semver-compatible "X.Y.Z-preview.N.gHASH".
 - Update autobuild.sh to generate filenames using the new scheme.
 - Update remove_old_firmware.py to match new scheme.
 - Update mpremote's pyproject.toml to handle the "-preview" suffix in the
   tag. setuptools_scm maps to this "rc0" to match PEP440.
 - Fix docs heading where it incorrectly said "vvX.Y.Z" for release docs.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>