Rename collectItems() and groupItems() into collect() and group()

[cescoffier]

It follows the same names as the select() group from #408.

• Deprecate previous methods and provide the new methods
• Update tests, documentation, and pictures
• • Remove the MultiCollect intermediate class

[codecov]

Codecov Report

Merging #413 (86e7b76) into master (044957b) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #413      +/-   ##
============================================
+ Coverage     89.24%   89.38%   +0.14%     
+ Complexity     2715     2713       -2     
============================================
  Files           363      362       -1     
  Lines         10524    10518       -6     
  Branches       1306     1308       +2     
============================================
+ Hits           9392     9402      +10     
+ Misses          588      577      -11     
+ Partials        544      539       -5     

Impacted Files	Coverage Δ	Complexity Δ
...tation/src/main/java/io/smallrye/mutiny/Multi.java	94.44% <100.00%> (+0.69%)	16.00 <2.00> (+2.00)
...n/java/io/smallrye/mutiny/groups/MultiCollect.java	100.00% <100.00%> (ø)	12.00 <10.00> (+2.00)
...io/smallrye/mutiny/groups/MultiGroupIntoLists.java	100.00% <100.00%> (ø)	6.00 <5.00> (ø)
...va/io/smallrye/mutiny/operators/AbstractMulti.java	100.00% <100.00%> (ø)	24.00 <2.00> (ø)
...rye/mutiny/streams/stages/CollectStageFactory.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
...e/mutiny/streams/stages/FindFirstStageFactory.java	100.00% <100.00%> (ø)	5.00 <0.00> (ø)
.../io/smallrye/mutiny/converters/multi/ToSingle.java	100.00% <100.00%> (ø)	3.00 <1.00> (ø)
...ye/mutiny/converters/multi/ToSingleFailOnNull.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...java/io/smallrye/mutiny/helpers/Subscriptions.java	80.11% <0.00%> (-1.11%)	48.00% <0.00%> (-1.00%)
...perators/multi/processors/SerializedProcessor.java	90.09% <0.00%> (ø)	24.00% <0.00%> (-1.00%)
... and 6 more