chore: clarify TODO about setting encryption parameters for volumeSourceType = backup is valid

[nschad]

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What this PR does / why we need it:

TODO removed. Implementation was correct, added some more input

Which issue(s) this PR fixes:
Fixes #

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Breaking changes:

[ske-prow]

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[nschad]

/cherry-pick release-v1.34
/cherry-pick release-v1.35
/cherry-pick release-v1.36

[stackit-ske]

@nschad: once the present PR merges, I will cherry-pick it on top of release-v1.34, release-v1.35, release-v1.36 in new PRs and assign them to you.

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In response to this:

/cherry-pick release-v1.34
/cherry-pick release-v1.35
/cherry-pick release-v1.36

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