Refactor Members ActivateService

What does this MR do and why?

Issue: #361096 (closed)

This refactors the ::Members::ActivateService by

1. Using factory methods to initialize the service
2. Use update_all instead of looping over each member
3. Make it clear that either an invited member, a user or the whole group gets activated.

This has the following advantages:

1. The API is easier to use and less error prone. Before that you had to either pass user, member or activate_all: true
2. We use an update_all instead of doing a looping over each Member
3. It's more clear what resource gets activates. Before this change, passing member did update all memberships of the given member's user.

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