fix(deps): remove unused dependency @backstage/backend-common

[knowacki23]

Hey, I just made a Pull Request!

This PR removes unused @backstage/backend-common from dependencies of the plugins.

✔️ 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-adr-backend	workspaces/adr/plugins/adr-backend	patch	v0.4.26
@backstage-community/plugin-search-backend-module-confluence-collator	workspaces/confluence/plugins/search-backend-module-confluence-collator	patch	v0.2.2
@backstage-community/plugin-copilot-backend	workspaces/copilot/plugins/copilot-backend	patch	v0.1.4
@backstage-community/plugin-vault-backend	workspaces/vault/plugins/vault-backend	patch	v0.4.20