docs: update release process for sub-modules

[LukeAVanDrie]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

This PR updates RELEASE.md to:

1. Address Issue Conformance module needs semver tag #2691 by adding explicit instructions for maintainers to tag sub-modules (e.g., conformance/go.mod) to support Go module pinning and downstream usage.
2. Document the usage of MAJOR=X MINOR=Y PATCH=Z make release to automate updating versions and Helm values across the repo.

Which issue(s) this PR fixes:

See #2691

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

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This pull-request has been approved by: LukeAVanDrie
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