prepare-branch does not work for RC2 and higher

[lidel]

Found while working on RC2 and RC3 of ipfs/kubo#10436

using ./kuboreleaser --skip-check-before release --version v0.30.0-rc2 prepare-branch did not bump CurrentVersionNumber in version.go in release-0.30.0

Same for v0.30.0-rc3, had to manually update CurrentVersionNumber in version.go in release-0.30.0.

Not a big deal, we rarely have more than one RC, but would be good to error and ask user to do the step, otherwise it is easy to miss and ship RC with broken ipfs version

(cc @galargh for visibility, in case @gammazero needs to make more than one RC for future 0.31.x)

[galargh]

We'll fix it as the first thing in October, would that work for you?

[lidel]

Yes, that works. Until them we will be following manual steps we have in kubo/docs/RELEASE_CHECKLIST.md