Warn only about latest stable release (non prerelease, draft)

[sisidra]

Currently developers receive message 'WARN com.spotify.scio.VersionUtil$ - A newer version of Scio is available: 0.7.4 -> v0.8.0-alpha1'

Warning level message about non-stable releases is not a desired behavior.

This attempts to remove such warnings.

This PR lacks tests...

[codecov]

Codecov Report

Merging #1980 into master will decrease coverage by 1.7%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1980      +/-   ##
==========================================
- Coverage   69.36%   67.65%   -1.71%     
==========================================
  Files         215      215              
  Lines        6672     6672              
  Branches      352      364      +12     
==========================================
- Hits         4628     4514     -114     
- Misses       2044     2158     +114

Impacted Files	Coverage Δ
.../src/main/scala/com/spotify/scio/VersionUtil.scala	80% <ø> (ø)	⬆️
...scala/com/spotify/scio/bigquery/BigQueryUtil.scala	50% <0%> (-50%)	⬇️
...la/com/spotify/scio/bigquery/client/QueryOps.scala	0.8% <0%> (-48.39%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.scala	25% <0%> (-32.5%)	⬇️
.../spotify/scio/bigquery/client/BigQueryConfig.scala	41.17% <0%> (-29.42%)	⬇️
...scala/com/spotify/scio/bigquery/client/Cache.scala	62.96% <0%> (-18.52%)	⬇️
...la/com/spotify/scio/bigquery/client/TableOps.scala	1.76% <0%> (-16.82%)	⬇️
...com/spotify/scio/bigquery/types/BigQueryType.scala	57.89% <0%> (-15.79%)	⬇️
...la/com/spotify/scio/bigquery/client/BigQuery.scala	28.94% <0%> (-10.53%)	⬇️
...n/scala/com/spotify/scio/bigquery/BigQueryIO.scala	24.66% <0%> (-2.67%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9fd56aa...78e33e6. Read the comment docs.