2.x: safeguard against building with v1 tags

[akarnokd]

This PR adds the same version guard that 1.x has: stop the build when the version tag doesn't start with "v2." to prevent releasing a 2.x version under the 1.x numbering.

This almost happened with today's botched 1.2.8 release where I forgot to set the target tag to 1.x, but noticed the mistake and stopped the build.

Since tags can't be deleted or retargeted, the fix can't prevent the wrong target tag mistake on itself. A small workaround is to mark bad tagged releases as pre-release.

Either way, the 2.x snapshots have a fixed 2.0.0-DP0-SNAPSHOT and don't draw a custom version number from the latest release tag on their branch, unlike 1.x.

[codecov]

Codecov Report

Merging #5219 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5219      +/-   ##
============================================
+ Coverage     96.07%   96.07%   +<.01%     
- Complexity     5748     5750       +2     
============================================
  Files           628      628              
  Lines         41085    41085              
  Branches       5698     5698              
============================================
+ Hits          39471    39473       +2     
+ Misses          648      639       -9     
- Partials        966      973       +7

Impacted Files	Coverage Δ	Complexity Δ
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.54% <0%> (-4.48%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	91.22% <0%> (-4.39%)	9% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-4.26%)	32% <0%> (-3%)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...activex/internal/operators/single/SingleUsing.java	97.59% <0%> (-2.41%)	4% <0%> (ø)
...main/java/io/reactivex/subjects/SingleSubject.java	97.61% <0%> (-2.39%)	39% <0%> (-1%)
...ternal/operators/completable/CompletableUsing.java	95.23% <0%> (-2.39%)	4% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	93.28% <0%> (-2.24%)	2% <0%> (ø)
... and 38 more

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[vanniktech]

Nice