Automatic merge from submit-queue

Support arbitrary alphanumeric strings as prerelease identifiers

**What this PR does / why we need it**: this is basically an extension of #43642, but supports more general prerelease identifiers, per the spec at http://semver.org/#spec-item-9.

These regular expressions are still a bit more restrictive than the SemVer spec allows (we disallow hyphens, and we require the format `-foo.N` instead of arbitrary `-foo.X.bar.Y.bazZ`), but this should support our needs without changing too much more logic or breaking other assumptions.

**Release note**:

```release-note
NONE
```