Enable -Xfatal-warnings for Scala 2.13

[cb372]

Now that those annotations have been deleted (#908), we can re-enable -Xfatal-warnings for Scala 2.13.x.

What this does?

Changes, features, fixes ...

Checklist

• Reviewed the diff to look for typos, println and format errors.
• Updated the docs accordingly.

[codecov]

Codecov Report

Merging #909 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #909   +/-   ##
=======================================
  Coverage   88.58%   88.58%           
=======================================
  Files          58       58           
  Lines         832      832           
  Branches        3        3           
=======================================
  Hits          737      737           
  Misses         95       95           

Impacted Files	Coverage Δ
...ess/mu/rpc/server/handlers/GrpcServerHandler.scala	100.00% <100.00%> (ø)

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