Rename occurrences of the master branch to main

[jponge]

Closes #496

[codecov]

Codecov Report

Merging #497 (056afad) into master (37def3f) will increase coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #497      +/-   ##
============================================
+ Coverage     90.08%   90.17%   +0.08%     
- Complexity     2933     2938       +5     
============================================
  Files           382      382              
  Lines         11340    11340              
  Branches       1420     1422       +2     
============================================
+ Hits          10216    10226      +10     
+ Misses          565      560       -5     
+ Partials        559      554       -5     

Impacted Files	Coverage Δ	Complexity Δ
...lrye/mutiny/subscription/SerializedSubscriber.java	83.01% <0.00%> (-5.67%)	26.00% <0.00%> (-2.00%)
...rye/mutiny/converters/uni/UniToMultiPublisher.java	88.88% <0.00%> (-2.78%)	2.00% <0.00%> (ø%)
...tiny/operators/multi/MultiBufferWithTimeoutOp.java	72.88% <0.00%> (-1.70%)	4.00% <0.00%> (ø%)
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	86.29% <0.00%> (+0.31%)	3.00% <0.00%> (ø%)
...erators/multi/builders/BufferItemMultiEmitter.java	92.10% <0.00%> (+1.31%)	28.00% <0.00%> (+1.00%)
...y/operators/multi/builders/IterableBasedMulti.java	84.04% <0.00%> (+2.12%)	4.00% <0.00%> (ø%)
.../io/smallrye/mutiny/helpers/queues/DrainUtils.java	86.66% <0.00%> (+8.88%)	14.00% <0.00%> (+2.00%)
...perators/multi/processors/SerializedProcessor.java	94.05% <0.00%> (+10.89%)	25.00% <0.00%> (+4.00%)