Update Dockerfile

[nopzdk]

updated spark version and modified download url to dlcdn for speedier download.

[tarzanek]

even upstream only supports 3.5.1
so I am not going to merge above, it would need more extensive testing, checking what changed if there aren't any incompats

@nopzdk can you take above change away?
I will be able to merge the rest

[tarzanek]

the spark upgrade has to be a new PR
we could try to get to 3.5.7, but besides the change you try here
we should change it for all testing ( so e.g. https://github.com/scylladb/scylla-migrator/blob/master/dockerfiles/spark/Dockerfile#L3 and then in all other places, even binaries depend on 3.5.1 - https://github.com/scylladb/scylla-migrator/blob/master/build.sbt#L5 , so spark upgrade HAS to be a separate change - I checked changelogs and for this repo the change seems OK, but we'd need first update https://github.com/scylladb/spark-scylladb-connector/blob/scylla-4.x/project/Versions.scala#L17 , along with jetty (it was updated in 3.5.x) - so there the change might be harder)