Setup release process and clean leftowers

[lukas-vlcek]

There are still some places where the code and configuration files need more love:

• build.gradle file (this file has been updated many times since the migration and I now consider it up-to-date)
• gradle.properties file (this file has been updated many times since the migration and I now consider it up-to-date)
• setup checkstyle (most of the work here has been already merged in Introduce indices filter #167, except it is turned off now)
• review CONTRIBUTING.md (see A long form of DCO signoff command is wrong. #199 and Add PR template to provide instructions how to add DCO #78)
• define security policy (see Specify Security Policy #19)
• verify trademarks and attributions (see Trademarks and Attributions cleansing #17)
• select Code of Conduct (see Specify Code of Conduct #15)
• review .github/CODEOWNERS (see Review CODEOWNERS #30)
• still some leftovers in tests from original plugin template (Rename*.java files) (see Removing "leftovers" from the plugin template. #191)
• final updates to README.md (there are some TODOs and TBDs in it) (see Prepare README for new release #27)
• rename the repository before it is moved to a new organisation (see here for more details about why the repository needs to change its name [PROPOSAL] Plugin naming conventions do not work well for community plugins opensearch-project/opensearch-plugin-template-java#23)

Deprecated tasks:

• missing automated release process: We will document the release process (see Document the release process #42) and automate later (if possible).