[release-v1.64] Use CreateOrGetAndStrategicMergePatch for BackupBucket generated secret in garden

[gardener-ci-robot]

This is an automated cherry-pick of #7708

/assign shafeeqes

A bug causing the gardenlet to be unable to access the BackupBucket generated secret in garden namespace is now fixed.

[shafeeqes]

/kind bug
/lgtm
/approve

[gardener-prow]

LGTM label has been added.

Git tree hash: b63aca01a84bd4b76d8d1fc259a12ab83ad361ef

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shafeeqes

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[gardener-prow]

@gardener-ci-robot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gardener-e2e-kind-ha-single-zone-upgrade-release-v1-64	ef5a6a0	link	false	/test pull-gardener-e2e-kind-ha-single-zone-upgrade-release-v1-64

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