Allow to adjust the Docker image tag for addons

[xrstf]

What this PR does / why we need it:
This is a proposal to help with the current problem surrounding addons in KKP.

It is currently very har to impossible to develop custom addons, as new Docker images cannot be easily refered to in userclusters. This is because the KKP Operator did not allow changing any Docker tags, because it wants to keep its own internal migration logic simple (so it does not allow to override the KKP version). However, this had a significant fallout for the addons, where there are legitimate usecases for overriding the Docker image.

This proposal tries to combine the best of both worlds. We still want to make sure admins don't accidentally upgrade KKP and continue to use their old addons image (which would be possible if we allowed to completely override the image tag), but we also want to allow to "bump" the tag to make development easier.

To achieve this, this PR simply introduces the concept of a "version suffix" that is, if set, appended to the KKP version.

Fixes #6081 in my opinion.

Does this PR introduce a user-facing change?:

Allow to customize the Docker image tag for Cluster Addons

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xrstf

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[zreigz]

yes, it solves the addon image problem. 👍
/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 7995c11160ec441cd3856b9f653272115e223dcb

[kubermatic-triage-bot]

/retest
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[kubermatic-bot]

@kubermatic-triage-bot:

In response to this:

/retest
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Review the full test history

Silence the bot with an /lgtm cancel or /hold comment for consistent failures.

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[xrstf]

/cherrypick release/v2.15

[kubermatic-bot]

@xrstf: new pull request created: #6106

In response to this:

/cherrypick release/v2.15

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