Update to GOV.UK Frontend 5.11 and release new version

[romaricpascal]

What

Update the version of GOV.UK Frontend to 5.11 once it's released and release a new version of the Gem offering the small fixes brought by 5.11 and the fixes added in #418 and #419

Why

So the latest fixes related to the brand refresh are available to users

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• We've updated gem to use 5.11
• We've released a new version on RubyGems
• We've communicated on Slack about the new version

[romaricpascal]

Closing as 5.0.2 uses GOV.UK Frontend 5.11