Update dependency wpackagist-plugin/activitypub to v5.1.0

This MR contains the following updates:

Package	Type	Update	Change
wpackagist-plugin/activitypub (source)	require	minor	5.0.0 -> 5.1.0

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This MR has been generated by Renovate Bot.

mentioned in issue #139

mentioned in commit 901b94f0

merged