Various roles depend on this. It makes sense to make it part
of the `matrix-base` role.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1682

Previously, when matrix-postgres was disabled, we were setting
`matrix_mautrix_twitter_database_engine` to an invalid empty value.

Now, we always hardcode `matrix_mautrix_twitter_database_engine: postgres`,
but set/unset the database hostname and password values instead.

Co-authored-by: Procuria <37988494+Procuria@users.noreply.github.com>

1.0.2 is the first container image tag that is available as a multi-arch image
with support for linux/amd64, linux/arm64/v8 (arm64) and linux/arm/v7 (arm32),
so self-building is no longer necessary on all these platforms.

4.95-r0-1 is the first container image tag that is available as a multi-arch image
with support for linux/amd64, linux/arm64/v8 (arm64) and linux/arm/v7 (arm32),
so self-building is no longer necessary on all these platforms.

2.2.3 is the first container image tag that is available as a multi-arch image
with support for linux/amd64, linux/arm64/v8 (arm64) and linux/arm/v7 (arm32),
so self-building is no longer necessary on all these platforms.

it's required if enabled by the dimension config here:
6eb8fb8392/roles/matrix-etherpad/tasks/init.yml (L42-L49)

see also #1517

related 0fb881deb5

see also #1527

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1517

Related to the work done in https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/818

Doing this further simplifies examples/vars.yml.

We're trying to move away from implementation-specific variables,
hoping for a clean (implementation-neutral) examples/vars.yml file.

Doing this seems more reasonable and simpler.

The goal is to have a single variable which tells us which homeserver
software is in use. Much simpler than having if/elif/elif checks for
variables like (`matrix_synapse_enabled` and `matrix_dendrite_enabled`, etc.)
everywhere.

This brings dendrite.yaml up to date and cleans things up a bit.

Discussed in https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1514