chore: add changeset files

Signed-off-by: Camila Belo <camilaibs@gmail.com>

.changeset/olive-cars-rescue.md

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+---
+'@backstage/plugin-search': patch
+---
+
+Create an experimental search plugin that is compatible with declarative integration system.

.changeset/purple-forks-fetch.md

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+---
+'@backstage/frontend-app-api': patch
+---
+
+Register default api implementations when creating declarative integrated apps.

.changeset/tasty-avocados-hope.md

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+---
+'@backstage/plugin-catalog': patch
+---
+
+Migrate catalog api to declarative integration system.

.changeset/tough-camels-enjoy.md

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+---
+'@backstage/frontend-plugin-api': patch
+---
+
+Add support for `SidebarGroup` on the sidebar item extension.