Allow to use higher versions of prometheus_client (#7780)

Fixes https://github.com/matrix-org/synapse/issues/7641

The package was pinned to <0.8.0 without an obvious reasoning with
7ad1d763
in https://github.com/matrix-org/synapse/pull/5636


while the version selection looks to just try to exclude an arbitrary
next minor version number that might introduce API breaking changes.
Selecting the next minor number might be a good conservative selection.

Downstream distributions already reported success patching out the version
requirements.

This also fixes the integration of upgraded packages into openSUSE packages,
e.g. for openSUSE Tumbleweed which already ships prometheus_client >= 0.8 .

Signed-off-by: Oliver Kurz <okurz@suse.de>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>