Update and fix checkstyle and javadoc plugins

[HeinrichAD]

A new PR as requested in https://github.com/plantuml/plantuml-server/pull/192#issuecomment-940198961

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This PR update the checkstyle and javadoc plugins. This PR is one part of #189.

Changes

• checkstyle mvn checkstyle:checkstyle mvn checkstyle:checkstyle-aggregate
  • 2.11 -> 3.1.2
  • Module LineLength outside TreeWalker Modul
  • delete RedundantThrows module, hence it was removed with version 6.2 see https://github.com/checkstyle/checkstyle/issues/473
  • sort in alphabetical order
• javadoc mvn javadoc:javadoc
  • 2.9.1 -> 3.3.1
  • fix documentation over classes It turns out that javadoc wants two stars at the beginning. Before: /* ... */ After: /** ... */
  • At least my compiler gives me warnings if I you @SuppressWarnings("serial") instead of @SuppressWarnings("SERIAL")
  • update links to use https if possible Note: License file also use https: https://www.gnu.org/licenses/gpl-3.0.txt
  • update project link http://plantuml.sourceforge.net -> https://plantuml.com
• README
  • remove travis CI badges and use new GitHub workflow badges
  • use own screenshot instead of ftomassettis one (same image different repository url)
  • fix typos also closes #165
  • adjust ducumentation about the non existing -readonly tag for docker also closes #182
  • add syntax highlighting as far as possible