Update dependency wpackagist-plugin/activitypub to v5.5.0

This MR contains the following updates:

Package	Type	Update	Change
wpackagist-plugin/activitypub (source)	require	minor	5.4.1 -> 5.5.0

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composer.json

@@ -113,7 +113,7 @@
         "wpackagist-plugin/event-organiser": "3.12.8",
         "noblogs/ai-authenticate-rest-api-plugin": "0.0.4",
         "wpackagist-plugin/multisite-custom-css": "1.0",
-        "wpackagist-plugin/activitypub": "5.4.1",
+        "wpackagist-plugin/activitypub": "5.5.0",
         "wpackagist-plugin/gallery-block-lightbox": "1.15",
         "wpackagist-plugin/unconfirmed": "1.3.7",
         "noblogs/wp-mat": "0.1.4",