Support Scala 2.13 / Spark 3.2; drop Scala 2.11 / Spark 2 support

[srowen]

Changes to ensure compatibility with Scala 2.13 and Spark 3.2.0.
Also, drop Spark 2 / Scala 2.11 support. (Spark 2 + Scala 2.12 should continue to work)

[codecov-commenter]

Codecov Report

Merging #564 (979fe3c) into master (80f8737) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
- Coverage   85.49%   85.45%   -0.05%     
==========================================
  Files          18       18              
  Lines         965      976      +11     
  Branches       92       90       -2     
==========================================
+ Hits          825      834       +9     
- Misses        140      142       +2     

Impacted Files	Coverage Δ
...a/com/databricks/spark/xml/XmlDataToCatalyst.scala	63.63% <0.00%> (-6.37%)	⬇️
...atabricks/spark/xml/parsers/StaxXmlGenerator.scala	92.15% <ø> (ø)
...in/scala/com/databricks/spark/xml/XmlOptions.scala	97.61% <0.00%> (-2.39%)	⬇️
.../scala/com/databricks/spark/xml/util/XmlFile.scala	100.00% <0.00%> (ø)
...abricks/spark/xml/parsers/StaxXmlParserUtils.scala	96.87% <0.00%> (+0.10%)	⬆️

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