feat(explore): remove support for old backend system

[secustor]

Hey, I just made a Pull Request!

This PR removes support for the old Backend system and removes the dependency to @backstage/backend-common

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-explore-backend	workspaces/explore/plugins/explore-backend	minor	v0.2.0

[awanlin]

Hi @secustor, thanks for this, just commenting around timing to start.

[github-actions]

This PR has been automatically marked as stale because it has not had recent activity from the author. It will be closed if no further activity occurs. If the PR was closed and you want it re-opened, let us know and we'll re-open the PR so that you can continue the contribution!

[awanlin]

Alright, let's move this forward @secustor, if you can resolve the conflict then we should be able to merge this in 👍

[awanlin]

Thanks again @secustor 🚀