Build tag configuration duplicated in many places

Currently the build tags get duplicated in a handful of places:

• Complement CI in GHA
• Complement CI in Buildkite
• Synapse CI in GHA
• Synapse complement.sh script
• Synapse CI in Buildkite (which is actually unused, but still sometimes kept in sync)
• Dendrite CI in Buildkite

Note that the build tags are homeserver specific (and might vary based on the version of the homeserver).

It would be great if complement was able to know which build tags to use somehow by inspecting a file or the homeserver capabilities. This would help avoid the various configs above getting out of sync.

Additionally we should probably:

• Remove the buildkite CI for complement.
• Remove the unused pipeline for Synapse.

Confronta le definizioni dei build tag in .github/workflows/ci.yaml, .github/workflows/tests.yml, scripts-dev/complement.sh e nei file delle pipeline di Complement, Synapse e Dendrite a cui si fa riferimento. Inizia mappando quali tag sono necessari per ogni versione dell'homeserver e dove vengono utilizzati. Il lavoro è completo quando le configurazioni rimangono sincronizzate tramite un'unica fonte facilmente individuabile o un controllo delle capacità, e le pipeline Buildkite inutilizzate sono state rimosse.