mautrix publishes each release under two tag schemes: v0.YYMM.PATCH
(also used for git tags, due to Go's module path requirements for
major versions >= 2) and a calver vYY.MM[.PATCH] scheme that exists
only on the Docker registry.

We switched mautrix-signal to the calver scheme in 3564155a7, which
left it silently stuck at v26.02.2: the calver tags have an
inconsistent number of components (v26.02.2 vs v26.05), and Renovate's
docker versioning only offers updates between tags with the same
number of dot-separated parts. It also broke self-building, which uses
the version as a git ref, and calver tags do not exist in git.

Going back to the v0 scheme (used by all other mautrix bridges) fixes
both problems and upgrades signal from the February release to the
current May one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>