Fix maven & gradle examples in the docs, enabling line-by-line display of test coverage in merge requests out-of-the-box (for java & kotlin)

What does this MR do?

Just as the commit messages state:

• Fix java and kotlin examples to work out-of-the-box
  This enables line-by-line display of test coverage in merge requests.
  As discovered in this discussion, a trailing slash together with a proper base prefix is required to make this work. Execution of source2filename.py script is no longer required since "Automatic classpath correction" was introduced for the gitlab cobertura parser.
• Remove single quotes from scripts in maven example
  Technically they aren't needed for single-line commands, while omitting them makes for a nicer code coloring in some IDEs (such as IntelliJ)
• Amend imperative to present tense in comment
  Reading the imperative "use" may imply that there is a TODO for users of this example, why there is none anymore, as this can now be used w/o further customizations and line-by-line coverage display in merge request should start working straightaway. Still keeping it there in descriptive, present-tense form of "using", so in case users want to experiment with it is still transparent that this path needs to be relative to the project root.

Related issues

Those issues are related, but not solved by this MR:

• #227345
• #331572

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