linguist-backend: Replace O(n) REST calls to catalog backend with sin…

[scott-kausler]

…gle call when cleaning up linguist entities by merging it with the add call.

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:heavy_check_mark: Checklist

• [X] A changeset describing the change and affected packages. (more info)
• [ ] Added or updated documentation
• [X] 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[bot]]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-linguist-backend	workspaces/linguist/plugins/linguist-backend	patch	v0.20.1