Changes: Fixed issues with the new maven builder; Changes on issue templates and more

[n1klasse]

Following changes:

Resources and code

• Added plugin.yml
• Added Support for 1.21.x

Maven builder

• Updated dependencies in pom.xml
• Added support that variables in plugin.yml get replaced with the right variables by the pom.xml
• Removed not used repositories in pom.xml

ISSUE_TEMPLATES

• Update links to the right wiki and discord server
• Issues are now much better to answer without irrelevant text fields
• Added version request

README and more

• Updated readme.md to fit to this repository
• Updated security.md to fit to this repository
• Added maven.yml as workflow for github actions
• Removed not used files

I hope this helps.

If some files should not be removed, just comment it below.

BTW: I don't know for what reason the .gitlab-ci.yml is used. But I left it where it is right now.

When this pull request is merged, the status of the issues will be changed:

closes #53
closes #58
closes #59
closes #90
closes #159
closes #191
resolves #110
resolves #206
resolves #237
resolves #236