Add -release 8 scalac option

[regadas]

Fixes #3000

This flag will force scala to target 8 to avoid the mentioned issue when compiling/publishing with jdk11. It doesn't play well in some other situations (read thread) and in our case I had to disabled it in scio-repl (ClassLoader issues) and while doing doc generation. In both these cases it's not really important that this flag is enabled.

[codecov]

Codecov Report

Merging #3006 into master will decrease coverage by 2.62%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3006      +/-   ##
==========================================
- Coverage   72.72%   70.09%   -2.63%     
==========================================
  Files         225      225              
  Lines        7498     7498              
  Branches      326      319       -7     
==========================================
- Hits         5453     5256     -197     
- Misses       2045     2242     +197     

Impacted Files	Coverage Δ
...scala/com/spotify/scio/bigquery/client/Cache.scala	0.00% <0.00%> (-74.08%)	⬇️
.../spotify/scio/bigquery/client/BigQueryConfig.scala	0.00% <0.00%> (-70.59%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.scala	0.00% <0.00%> (-52.50%)	⬇️
...scala/com/spotify/scio/bigquery/BigQueryUtil.scala	50.00% <0.00%> (-50.00%)	⬇️
...otify/scio/bigquery/syntax/ScioContextSyntax.scala	19.35% <0.00%> (-41.94%)	⬇️
...la/com/spotify/scio/bigquery/client/QueryOps.scala	0.73% <0.00%> (-39.71%)	⬇️
.../scala/com/spotify/scio/bigquery/StorageUtil.scala	0.00% <0.00%> (-36.85%)	⬇️
...com/spotify/scio/bigquery/types/BigQueryType.scala	58.33% <0.00%> (-29.17%)	⬇️
...la/com/spotify/scio/bigquery/client/TableOps.scala	0.00% <0.00%> (-25.00%)	⬇️
...com/spotify/scio/bigquery/types/TypeProvider.scala	52.11% <0.00%> (-22.54%)	⬇️
... and 2 more

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