bump: Scala XML 2.1.0 across the modules

[johanandren]

No description provided.

[ennru]

Twirl 1.5.2 was released with Scala XMl 2.1.0 https://github.com/playframework/twirl/releases/tag/1.5.2

[johanandren]

Next problem(s):

[error] 	* org.scala-lang.modules:scala-xml_2.12:2.1.0 (early-semver) is selected over {1.0.6, 1.3.0}
[error] 	    +- org.scala-lang:scala-compiler:2.12.17              (depends on 2.1.0)
[error] 	    +- com.typesafe.play:twirl-api_2.12:1.5.2             (depends on 2.1.0)
[error] 	    +- ws.unfiltered:unfiltered_2.12:0.10.4               (depends on 1.3.0)
[error] 	    +- org.foundweekends:knockoff_2.12:0.8.6              (depends on 1.0.6)

[ennru]

These dependencies come from sbt-site which pulls in all site generators. That's why I tried to pull it apart in sbt/sbt-site#203

[ennru]

LGTM.