Update default branch in workflows

[ohltyler]

Issue #, if available:

Description of changes:

With the recent efforts to change the default branch name from master to main, some workflows had master hardcoded - this changes those to main instead.

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[codecov]

Codecov Report

Merging #387 (346665f) into main (a6ce6fc) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #387      +/-   ##
============================================
+ Coverage     79.07%   79.09%   +0.01%     
- Complexity     2658     2661       +3     
============================================
  Files           247      247              
  Lines         11717    11717              
  Branches       1008     1008              
============================================
+ Hits           9265     9267       +2     
  Misses         1971     1971              
+ Partials        481      479       -2     

Flag	Coverage Δ	Complexity Δ
plugin	79.07% <ø> (+0.01%)	0.00 <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...port/SearchAnomalyDetectorInfoTransportAction.java	54.54% <0.00%> (-9.10%)	4.00% <0.00%> (ø%)
...sticsearch/ad/indices/AnomalyDetectionIndices.java	50.94% <0.00%> (-0.32%)	44.00% <0.00%> (-1.00%)
...ndistroforelasticsearch/ad/task/ADTaskManager.java	73.05% <0.00%> (+0.22%)	82.00% <0.00%> (+1.00%)
...on/opendistroforelasticsearch/ad/model/ADTask.java	96.56% <0.00%> (+1.25%)	66.00% <0.00%> (+2.00%)
...ansport/handler/AnomalyResultBulkIndexHandler.java	89.79% <0.00%> (+4.08%)	13.00% <0.00%> (+1.00%)