[Meta] Add automated license header check to all plugins

[dblock]

Is your feature request related to a problem?
Some obsolete ALv2 headers are still found in various plugins, e.g. here.

What solution would you like?

1. Document an SOP of how to add automated license checks for both OpenSearch and OpenSearch Dashboards. One possible solution is to add automated license header Gradle checks to CI similar to this.
2. Update the default header for all new files created by IDEs similar to [License] Update SPDX License Header OpenSearch#510.
3. Fix any incorrect headers.

Do you have any additional context?
Licenses must match https://github.com/opensearch-project/opensearch-plugins/blob/main/HEADERS.md

• [BUG] Incorrect license headers security#1744

[ohltyler]

It seems this has been punted since 2.1 - is there any progress on this campaign actually being enforced for 2.5 or 2.6? It seems misleading to not follow through on campaigns listed in the main release issues.