Use the simplecov 1.x spellings, and unstick the client base image

[JonJagger]

The base image now carries simplecov 1.1.1 where it carried 0.21.2.
Four spellings in the test harness are deprecated there, each announcing
itself on stderr on every run:

add_group  -> group
add_filter -> skip
# :nocov:  -> # simplecov:disable / # simplecov:enable

and the formatter reopened SimpleCov::Formatter::JSONFormatter to
redefine format, which in 1.1.1 makes ruby -w report the redefinition.
It is now CoverageMetricsFormatter, named for the coverage_metrics.json
it writes. It never needed to be that class: what it produces is
per-group totals, not the per-file shape the shipped formatter writes,
so it was only borrowing the name to make itself win.

The client tests then failed where the server tests passed, because
source/client/Dockerfile was pinned to cyberdojo/sinatra-base:759c4e9 on
Docker Hub, while everything else moved to ghcr.io. The automated
base-image PR only rewrites the Dockerfile at the repo root, so that pin
had gone unbumped long enough to be serving ruby 3.3 and simplecov
0.21.2. Both Dockerfiles now name the same image, so both suites see the
same gems, and the two coverage.rb copies can hold the same spellings.

The group block parameter goes from the to path while passing, matching
the other repos and saying what it is.

Coverage is unchanged either side: server test 375 / code 253, client
test 196 / code 80, nothing missed in any of them.