changesets: add changesets for v1 extension removal

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/brown-boxes-arrive.md

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+---
+'@backstage/frontend-plugin-api': minor
+---
+
+**BREAKING**: Removed support for "v1" extensions. This means that it is no longer possible to declare inputs and outputs as objects when using `createExtension`. In addition, all extension creators except for `createComponentExtension` have been removed, use the equivalent blueprint instead. See the [1.30 migration documentation](https://backstage.io/docs/frontend-system/architecture/migrations/#130) for more information on this change.

.changeset/empty-coats-sparkle.md

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+---
+'@backstage/frontend-test-utils': minor
+---
+
+Removed support for testing "v1" extensions, where outputs are defined as an object rather than an array.

.changeset/serious-cheetahs-help.md

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+---
+'@backstage/plugin-catalog-react': patch
+'@backstage/plugin-search-react': patch
+'@backstage/plugin-catalog': patch
+---
+
+The `/alpha` export no longer export extension creators for the new frontend system, existing usage should be switched to use the equivalent extension blueprint instead.

.changeset/sharp-mayflies-beg.md

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+---
+'@backstage/frontend-app-api': patch
+---
+
+Internal refactor following removal of v1 extension support. The app implementation itself still supports v1 extensions at runtime.