Switch back to using Sytest dinsic branch

As part of the 1.31.0 mainline merge, we updated the buildkite pipeline to
more closely match mainline. Unfortunately we accidentally also updated the
sytest portion verbatim, which meant we were now running Sytest develop
against dinsic. This inevitably broke as dinsic started to fall behind
mainline again.

This commit returns us back to the dinsic tag of matrixdotorg/sytest-synapse,
as well as removes some Linux distro variations which we aren't using
for dinsic.

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