Remove legacy code

Deletes most legacy code from the code base.

The legacy code will remain in the branch main-compat-edition.

Not removed so far:

• Some tests for which there is no replacement yet
• The /_eliatra/security/api/license action which might be still used by the Dashboards plugin (needs to be clarified)

Two JUnit tests have been set to @Ignore due to failures and must be investigated soon:

• https://git.eliatra.com/eliatra-suite/eliatra-opensearch-plugin/-/issues/47
• https://git.eliatra.com/eliatra-suite/eliatra-opensearch-plugin/-/issues/48

The int tests need to be adapted separately.

Closes #7 (closed), #31 (closed)