Experiment - Update new user invitation email with activity

What does this MR do?

• Removes variants for the experiment as per https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/325#note_594837753
  • making this part of the same MR as adding a variant makes it easier to adjust the experiment and re-use existing code.
• Adds group/project activity variant to the experiment as per https://gitlab.com/gitlab-org/gitlab/-/issues/330353

How to test?

• In Rails console(bundle exec rails console), issue this command to put yourself in the experiment

Feature.enable(:members_invite_email)

• send an email 2 times to cycle through the variants. May need to send a few invites before the cache clears and updates sidekiq to see the change on feature flag state changes.
• To completely disable the experiment, disabled the feature flag.

Feature.disable(:members_invite_email)

Screenshots (strongly suggested)

With new experiment variant for group/project activity

with new experiment variant for group/project activity + truncation of text as per design specs

Summary of changes

source	variant	email
project	control(no change)
project	activity(Added in this MR)
project	permission_info(Removed in this MR)
project	avatar(Removed in this MR)
group	control(no change)
group	activity(Added in this MR)
group	permission_info(Removed in this MR)
group	avatar(Removed in this MR)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #330353