Revise the version regex in GsonVersionDiagnosticsTest.

[eamonnmcmanus]

Before we can release 2.10 we must support two-digit components. Additionally, there's no reason to require a patch number (2.10.0 rather than 2.10).

[Marcono1234]

If we were strictly following Semantic Versioning then the patch version is required:

A normal version number MUST take the form X.Y.Z where X, Y, and Z are non-negative integers, and MUST NOT contain leading zeroes.

Though I am not sure if that matters much. Also thanks a lot for creating the release!

Edit: And omitting 0 as patch version seems to match Guava's versioning scheme.

[eamonnmcmanus]

Hmm, yes, starting with 2.8.0 in 2016 we've always included the trailing .0, so it was a mistake to have this release be 2.10 rather than 2.10.0. I think I'll undo that part of this PR and add a comment.

[eamonnmcmanus]

Sent #2237 to fix the test and prepare for following the SemVer spec strictly again.