Handle leading "v" in versions in a commit message

[jonmcquillan]

eg Bumps org/repo from v1.3.0 to v1.3.2. as a commit message does not populate the previous-version and new-version, so is also unable to calculate the proper update-type. This makes the output not very useful for determining whether to auto merge a dependabot PR only on minor version changes or less for example.

[jonmcquillan]

Something like this maybe? #245

[jeffwidman]

Makes sense, this should get fixed.

Input: Bumps org/repo from v1.3.0 to v1.3.2

Expected output:
previous-version: "v1.3.0"
new-version: "v1.3.2"
update-type: "version-update:semver-patch"

In other words, the "handling" should strip the leading v when calculating the update-type, but not actually strip it when populating the final "previous-version" / "new-version"...

That way the behavior is consistent between the version coming from the library and the action so users don't have to translate back and forth if they do additional processing.