chore: deprecate createRouter and RouterOptions in plugins

[PranavKasetty]

Hey, I just made a Pull Request!

Deprecated the createRouter and RouterOptions as part of #26353

• example-todo-list-backend
• notifications-backend
• signals-backend
• user-settings-backend

✔️ 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
@internal/plugin-todo-list-backend	plugins/example-todo-list-backend	none	v1.0.33-next.2
@backstage/plugin-notifications-backend	plugins/notifications-backend	patch	v0.4.3-next.2
@backstage/plugin-signals-backend	plugins/signals-backend	patch	v0.2.3-next.2
@backstage/plugin-user-settings-backend	plugins/user-settings-backend	patch	v0.2.27-next.2

[backstage-goalie]

Thanks for the contribution!
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[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!