CI: checkstyle rule for Java file headers: additional fixes

[algattik]

What this PR changes/adds

This PR modifies some Java source files to enable the checkstyle header check rule in #997.

Why it does that

Reduce the developer overhead of fixing checkstyle header issues after merging #997.

Further notes

• CyclicDependencyException.java: removed Jetty notice since the class is a generic implementation of EdcException
• TopologicalSort.java: moved Jetty notice down so that standard header goes first
• DataRequestMessageSender.java: deleted empty file
• JsonFileGenerator.java: moved out of default package. This will also allow to create a package name rule later

Also fixed an unrelated merge remnant marker in a README file.

Linked Issue(s)

Relates to #991

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• added relevant details to the changelog? (skip with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)

[codecov-commenter]

Codecov Report

Merging #1042 (9d0328d) into main (8da8f4c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1042   +/-   ##
=========================================
  Coverage     64.57%   64.57%           
  Complexity     2862     2862           
=========================================
  Files           671      671           
  Lines         14733    14733           
  Branches       1013     1013           
=========================================
  Hits           9514     9514           
  Misses         4755     4755           
  Partials        464      464           

Impacted Files	Coverage Δ
...connector/boot/util/CyclicDependencyException.java	100.00% <ø> (ø)
.../dataspaceconnector/boot/util/TopologicalSort.java	92.85% <ø> (ø)

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