[Task]: Update docs for RC image tags

[johannaojeling]

What needs to happen?

The container image tags for RCs follow the pattern ${RELEASE_VERSION}rc{RC_NUM} instead of ${RELEASE_VERSION}_rc{RC_NUM} since v2.40.0. Update the release guide and blog post to reflect this.

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

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[johannaojeling]

.take-issue