Upgrade ScyllaDB version

[mykaul]

Version used currently is a bit outdated (end-of-life actually):

janusgraph/pom.xml

Line 107 in 94c55f0

<scylladb.version>5.1.4</scylladb.version>

Should use 6.1.0 or above, for latest version.
See https://forum.scylladb.com/t/relase-scylladb-6-1/2486

[mykaul]

CC @FlorianHockmann

[FlorianHockmann]

Just to be clear: This is only the version we are testing against. Users have to set up their own Scylla cluster and are therefore not bound by this. But nevertheless we should of course update this so we test against a version that is still supported.