Add Trio-Specific App Group for Isolated Data Storage

[dnzxy]

This pull request addresses the issue of shared app group data conflicts (addresses and solves #313). It introduced a Trio-specific app group: group.org.nightscout.YOUR-TEAMID.trio.trio-app-group.

Changes:

• Add new user-defined build setting APP_GROUP_ID with value group.$(BUNDLE_IDENTIFIER).trio-app-group to Trio scheme.
• Update the xcconfig file to removeAPP_GROUP_ID as its now set via build settings.
• Modify testflight.md to document the new Trio app group setup.

Benefits:

• Enhanced data integrity and security.
• Clear separation of app data, reducing risks of conflicts.

Implications:

• Users will not lose a pod or other paired devices.
• No impact on Xcode builders for fresh downloads.
• For Xcode builders with existing downloads, users may need to click on their build targets (will show a provisioning profile error) to trigger a refresh of app group profiles
• New Browser builders need to create and use the new Trio-specific app group instead of LoopGroup.
• Existing Browser builders need to create a new app group, use that instead of LoopGroup and subsequently run the 3. Create Certificate and 4. Build Trio actions.
• Xcode builders transitioning to Browser build may want to follow the instructions in the new section Optional: Description Modification in testflight.md to rename the display name of the app group on Apple Developer for more clarity.

Breaking Changes:

• Adopting this change will break current compatibility of Trio with xDrip4iOS and GlucoseDirect as CGM sources
• This PR will stay in draft mode for the time being until the maintainers of xDrip4iOS have had the chance to make small changes to accommodate the changed app group and give users the functionality to choose between apps (i.e. app groups) to share data with
• xDrip4iOS will add a new "Loop Share" workflow with its upcoming v5.3 release. There's a pending staging PR (see xdripswift#551) opened now.
• GlucoseDirect seems to no longer be maintained; the project could be forked and slightly adapted if the need arises.

[marionbarker]

See testing done with xDrip4iOS at this comment:

• https://github.com/JohanDegraeve/xdripswift/pull/551#issuecomment-2201231956
• That testing was with the pbxproj change - not with this PR

I will test this PR after the inadvertent CGMBLEKit change is removed.