[Javadocs] add to o.o.action.admin

[nknize]

Adds javadocs to classes in package o.o.action.admin and removes unused classes. Also
adds custom javadoc tags opensearch.internal, opensearch.experimental, and opensearch.api

relates #221
relates #2868

[opensearch-ci-bot]

❌   Gradle Check failure f9da54ce4d55e49b16ee034e2039b5d030a227d3
Log 4934

Reports 4934

[opensearch-ci-bot]

❌   Gradle Check failure 2733632ec048a1b8dfb39bcc70ad8a341295a31e
Log 4936

Reports 4936

[opensearch-ci-bot]

❌   Gradle Check failure df8cc9cf34e4620c29c7ffbb25df4baa158cabfd
Log 4940

Reports 4940

[opensearch-ci-bot]

✅   Gradle Check success 2f93a2721e3e0510789f4ce0afdc979c4a91dd11
Log 4944

Reports 4944

[reta]

@nknize do you think we could move these changes to separate pull request (as you did with aliases & types)?

[nknize]

Sure thing!

[opensearch-ci-bot]

✅   Gradle Check success 0982d86
Log 4962

Reports 4962

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-3155-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ad1c8038b01d6d82e5393d73bcbf28a43bb97bc2
# Push it to GitHub
git push --set-upstream origin backport/backport-3155-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-3155-to-2.x.

[opensearch-trigger-bot]

The backport to 2.0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.0 2.0
# Navigate to the new working tree
cd .worktrees/backport-2.0
# Create a new branch
git switch --create backport/backport-3155-to-2.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ad1c8038b01d6d82e5393d73bcbf28a43bb97bc2
# Push it to GitHub
git push --set-upstream origin backport/backport-3155-to-2.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.0

Then, create a pull request where the base branch is 2.0 and the compare/head branch is backport/backport-3155-to-2.0.