Change $default_branch to static branches (and set Java version correctly)

[HyukjinKwon]

This PR proposes to use:

• static branches instead of $default-branch that's to be used as a template.
• use adopt@1.8 syntax (as documented in https://github.com/olafurpg/setup-scala). Otherwise it picks JDK 11 for java-version: '8'.

[HyukjinKwon]

okay seems working fine @srowen :-)

[codecov-commenter]

Codecov Report

Merging #545 (95d3551) into master (12a2583) will increase coverage by 0.01%. The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #545      +/-   ##
==========================================
+ Coverage   86.04%   86.05%   +0.01%     
==========================================
  Files          18       18              
  Lines         953      954       +1     
  Branches       78       95      +17     
==========================================
+ Hits          820      821       +1     
  Misses        133      133              

Impacted Files	Coverage Δ
...scala/com/databricks/spark/xml/util/TypeCast.scala	87.28% <89.47%> (+0.33%)	:arrow_up:
...a/com/databricks/spark/xml/XmlDataToCatalyst.scala	70.00% <0.00%> (-5.00%)	:arrow_down:

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[srowen]

Nice, thank you!