add Dependabot

[sullis]

https://github.blog/2020-06-01-keep-all-your-packages-up-to-date-with-dependabot/

[codecov-commenter]

Codecov Report

Merging #2210 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2210      +/-   ##
============================================
- Coverage     83.82%   83.80%   -0.02%     
  Complexity     4151     4151              
============================================
  Files           380      380              
  Lines         30918    30918              
  Branches       5723     5723              
============================================
- Hits          25918    25912       -6     
- Misses         3444     3447       +3     
- Partials       1556     1559       +3     

Impacted Files	Coverage Δ	Complexity Δ
...in/java/reactor/core/publisher/FluxWindowWhen.java	84.13% <0.00%> (-1.45%)	2.00% <0.00%> (ø%)
...n/java/reactor/core/publisher/ReplayProcessor.java	87.50% <0.00%> (-1.39%)	40.00% <0.00%> (-1.00%)
...eactor/core/publisher/FluxSubscribeOnCallable.java	72.50% <0.00%> (-0.84%)	3.00% <0.00%> (ø%)
...main/java/reactor/core/publisher/FluxGenerate.java	82.82% <0.00%> (-0.62%)	7.00% <0.00%> (ø%)
...actor/core/publisher/FluxOnBackpressureBuffer.java	93.71% <0.00%> (-0.58%)	4.00% <0.00%> (ø%)
...eactor/core/publisher/ParallelMergeSequential.java	84.97% <0.00%> (-0.52%)	7.00% <0.00%> (ø%)
...ain/java/reactor/core/publisher/FluxConcatMap.java	90.83% <0.00%> (+0.55%)	7.00% <0.00%> (ø%)
.../java/reactor/util/concurrent/MpscLinkedQueue.java	89.47% <0.00%> (+1.31%)	18.00% <0.00%> (+1.00%)

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[simonbasle]

Dependabot doesn't seem to be able to bump versions from gradle.properties, which we use (notably in preparation for more automation around releases). see dependabot/dependabot-core#1618