docs: add changeset files

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

.changeset/green-carpets-smell.md

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+---
+'@backstage/backend-plugin-api': patch
+---
+
+The `RootLifecycleService` now has a new `addBeforeShutdownHook` method, and hooks added through this method will run immediately when a termination event is received.

.changeset/large-birds-watch.md

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+---
+'@backstage/backend-test-utils': patch
+---
+
+Mock the new `RootLifecycleService.addBeforeShutdownHook` method.

.changeset/pretty-kiwis-travel.md

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+---
+'@backstage/backend-defaults': patch
+---
+
+Implements the `DefaultRootLifecycleService.addBeforeShutdownHook` method, and updates `DefaultRootHttpRouterService` and `DefaultRootHealthService` to listen to that event to stop accepting traffic and close service connections.

.changeset/young-cobras-add.md

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+---
+'@backstage/backend-app-api': patch
+---
+
+As soon as a backend termination signal is received, call before shutting down root lifecycle hooks.