Remove ReadOnly flag from backupPod Volumes spec

[shubham-pampattiwar]

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[weshayutin]

@shubham-pampattiwar @sseago nice work!

[sseago]

@Lyndon-Li this is the change that fixed backup on OpenShift without requiring privileged mode. ReadOnly mount is still preserved since the volumeMount entry still has ReadOnly with this change.

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##             main    #8248   +/-   ##
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[Lyndon-Li]

@sseago @shubham-pampattiwar
See my comments here #8243 (comment). Let's do some further checks since the readOnly flags in volumeSource and volumeMount are for different purposes and literally should be both set.

[sseago]

Closing in favor of #8250