3.x Switch from Travis CI to GitHub Actions

[akarnokd]

Recently, Travis CI introduced a pricing model that requires OSS projects to repeatedly apply for "free build credits". After consulting the internet, I decided to switch to GitHub Actions.

GitHub Actions (GHA) uses a build system similar to Travis CI but their setup is slightly different and the logic in .travis.yml and in various other files have to be converted to it.

This PR introduces 4 actions depending on how the project is interacted with:

• PR builds: used when someone posts a PR.
• Branch builds other than 3.x, used by dependabot or temporary branches.
• 3.x pushes: these generate snapshots
• releases: when a new release is created

The latter two require secret information such as login credentials to Bintray, Sonatype and GitHub itself (for Javadocs pushback) and these have to be specified on the project's secrets page:

• BINTRAY_USER (snapshot deploy)
• BINTRAY_KEY (snapshot deploy)
• SONATYPE_USER (release deploy)
• SONATYPE_PASSWORD (release deploy)
• JAVADOCS_TOKEN (used for pushing back the generated snapshot/release javadocs to gh-pages)

Unfortunately, to verify these actions, I have to merge stuff into 3.x to trigger snapshot and do an release candidate to trigger the release.

[codecov]

Codecov Report

Merging #7114 (d9908e8) into 3.x (5b408f6) will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7114      +/-   ##
============================================
+ Coverage     99.54%   99.59%   +0.04%     
- Complexity     6670     6671       +1     
============================================
  Files           742      742              
  Lines         47275    47275              
  Branches       6374     6374              
============================================
+ Hits          47061    47084      +23     
+ Misses           97       88       -9     
+ Partials        117      103      -14     

Impacted Files	Coverage Δ	Complexity Δ
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	10.00% <0.00%> (-1.00%)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0.00%> (-3.97%)	2.00% <0.00%> (ø%)
...nternal/operators/observable/ObservableCreate.java	96.58% <0.00%> (-3.42%)	2.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	96.82% <0.00%> (-1.59%)	2.00% <0.00%> (ø%)
...3/internal/operators/flowable/FlowableGroupBy.java	85.02% <0.00%> (-0.30%)	3.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableCreate.java	99.02% <0.00%> (+0.32%)	6.00% <0.00%> (ø%)
...nternal/operators/observable/ObservableReplay.java	100.00% <0.00%> (+0.53%)	22.00% <0.00%> (ø%)
.../reactivex/rxjava3/observers/BaseTestConsumer.java	100.00% <0.00%> (+0.55%)	70.00% <0.00%> (+1.00%)
.../operators/observable/ObservableCombineLatest.java	100.00% <0.00%> (+0.61%)	6.00% <0.00%> (ø%)
...operators/observable/ObservableMergeWithMaybe.java	100.00% <0.00%> (+0.90%)	2.00% <0.00%> (ø%)
... and 11 more

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