Refactor gardener-seed-admission-controller

[timebertt]

How to categorize this PR?

/area quality dev-productivity
/kind cleanup test
/priority normal

What this PR does / why we need it:

This PR refactors/enhances several things regarding the seed-admission-controller:

• migrate the component to a full controller-runtime component (only usage of manager was left)
• enable both admission review versions (including admission/v1) in the webhook configs, as it is now supported by the c-r webhook handlers
• decrease the timeouts of the extension deletion protection webhook to 10s according to best pratices
• add an envtest-style integration test for the extension deletion protection webhook
• migrate the handcrafted integration test for the scheduler name webhook to an envtest-style test

Which issue(s) this PR fixes:
Part of #3109

Special notes for your reviewer:

The PR got a bit bigger than expected, but it's still split into several commits, so I hope this helps with the review.
/squash

Release note:

`gardener-seed-admission-controller`'s webhooks now also accept reviews in version `admission/v1`. 
Also, webhook timeouts have been lowered to `10s` for the extension deletion protection webhooks.

[rfranzke]

/assign
/rebase

[rfranzke]

/lgtm

Very nice, thanks for adding the integration test!

[stoyanr]

/lgtm

I have a general question: I used to write envtest while working on other projects in the past (SEN). I think it's the first such test added to g/g, possibly because it requires a controller runtime component (is this correct?). Are there plans to migrate other components (e.g. gardenlet) to controller runtime as well, so we could write add such tests for them as well?

[timebertt]

I think it's the first such test added to g/g, possibly because it requires a controller runtime component (is this correct?).

It's not the first one, but the second (#2830 was the first one) 😉
You're right though, there are not many envtests in g/g currently.
I think, we should be able to leverage envtest for integration tests for some of our controllers as well, at least for the ones that already implement the reconciler.Reconcile interface. Though, we haven't done any of those yet.

Are there plans to migrate other components (e.g. gardenlet) to controller runtime as well, so we could write add such tests for them as well?

I plan to do the refactoring for gardener-admission-controller this week (similar to this PR).
I think, the general goal is to do the same for the controller components as well, though it will be much more effort and might reveal some difficulties. As of now, there are no concrete plans for this, I guess.

[timebertt]

Rebased to resolve conflicts.