Bump bytecode to 11

[nedtwigg]

We can start using Java 11 API and deps now that we've published a release (plugin-gradle 6.14.0 and plugin-maven 2.31.0) which can run on Java 8 just enough to yell at the user about updating.

• closes Bump minimum required JRE to 11? #1337

[simschla]

@nedtwigg I think it might be necessary to update these lines too:

spotless/plugin-maven/src/test/resources/pom-test.xml.mustache

Lines 18 to 19 in aa78b56

	<maven.compiler.source>1.8</maven.compiler.source>
	<maven.compiler.target>1.8</maven.compiler.target>

spotless/plugin-maven/src/test/resources/pom-test-management.xml.mustache

Lines 18 to 19 in aa78b56

	<maven.compiler.source>1.8</maven.compiler.source>
	<maven.compiler.target>1.8</maven.compiler.target>

spotless/plugin-maven/src/test/resources/multi-module/pom-parent.xml.mustache

Lines 14 to 15 in aa78b56

	<maven.compiler.source>1.8</maven.compiler.source>
	<maven.compiler.target>1.8</maven.compiler.target>

Or are they irrelevant?

[nedtwigg]

Thanks for reviewing this!

Those lines mean that mvn compile will produce a jar that can run on JRE 8, even if the JDK doing the build is anything 8+.

https://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-source-and-target.html