replace URL() constructors

[vladak]

This change gets rid of the URL() constructors deprecated in Java 20 and addresses a case of unescaped patterns.

Background: https://gist.github.com/vladak/beb43b9056b546869196d09a16542466

Approaches #4459 and takes over the URL() changes from @tarzanek's PR #4570

I had to add a flake8 suppression to the Docker entry point to make the build pass.

Tested with this read-only configuration:

<?xml version="1.0" encoding="UTF-8"?>
<java version="1.8.0_121" class="java.beans.XMLDecoder">
 <object class="org.opengrok.indexer.configuration.Configuration">

  <void property="bugPage">
   <string>https://github.com/apache/lucene/issues/</string>
  </void>

  <void property="bugPattern">
   <string>LUCENE-(\d+)</string>
  </void>

 </object>
</java>

and The Lucene Git repository.

[ginoaugustine]

LGTM