Update Ruby deps

[peterbroadhurst]

For https://github.com/hyperledger/firefly/security/dependabot/19

[nguyer]

Looks like one of the plugins we were using to check for broken links wasn't happy with the change. Maybe the flag is different in this version?

Run bundle exec htmlproofer --disable-external --allow-hash-href --assume-extension ./_site --url-swap '^/firefly/:/' --url-ignore /127.0.0.1/,/localhost/
/home/runner/work/firefly/firefly/docs/vendor/bundle/ruby/3.0.0/gems/mercenary-0.3.[6](https://github.com/hyperledger/firefly/runs/7870490745?check_suite_focus=true#step:13:7)/lib/mercenary/program.rb:31:in `go': invalid option: --url-swap (OptionParser::InvalidOption)
htmlproofer 4.3.2 | Error:  Whoops, we can't understand your command.

[peterbroadhurst]

Ok - after an annoying amount of faff, I've worked this out. A bunch of changes needed to the new command line.