Improve container runtime handling when machine image changes

[grolu]

What this PR does / why we need it:

• When selecting a machine image that does not support the currently selected container runtime, the Dashboard does no longer implicitly set the container runtime to something else
• If the user switches back to another image that supports the previously selected runtime it is restored
• This also applies for new clusters, I think it makes sense as the user may not notice it otherwise
• Container Runtime is now a required field also for not new clusters. There might still exist some very old worker groups without a container runtime set explicitly. However, modifying those would cause Gardener to add this now required field automatically anyway

Which issue(s) this PR fixes:
Fixes #1111

Special notes for your reviewer:

Release note:

When selecting a machine image that does not support the currently selected container runtime, the Dashboard does no longer implicitly set the container runtime to something else

[gardener-robot]

@holgerkoser You have pull request review open invite, please check

[petersutter]

/lgtm