Show one generic message in case HPPS functionality is disabled

[merkushin]

Resolves #7362

Proposed Changes

• Show a generic message when HPPS is disabled.

Testing Instructions

1. Add 'set_time_limit' to disable_functions in your php.ini (Like that: disable_functions = set_time_limit, get_resources)
2. Enable experimental UI feature flag: add_filter( 'sensei_feature_flag_experimental_features_ui', '__return_true' );
3. Go to Sensei LMS > Settings > Experimental Features.
4. Make sure you see a message like in the screenshot below.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[merkushin]

I suppose the failing checks are related to this PR: #7369

[donnapep]

Could we simplify this:

This feature is currently in development and should be used with caution. While HPPS is in its early experimental stage, the functionalities may be temporarily unavailable.

to something like this:

As this feature is currently experimental, it may not be available yet on some sites.

I think being in development and using "caution" are already implied by "experimental".

[merkushin]

@donnapep

Here is the updated message:

The second line ("As this feature is currently experimental, it may not be available yet on some sites.") appears only when the feature is disabled.

[codecov]

Codecov Report

Merging #7372 (c911025) into trunk (8f61415) will increase coverage by 0.02%.
Report is 20 commits behind head on trunk.
The diff coverage is 0.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk    #7372      +/-   ##
============================================
+ Coverage     50.74%   50.77%   +0.02%     
+ Complexity    11148    11145       -3     
============================================
  Files           613      613              
  Lines         47060    47042      -18     
  Branches        404      404              
============================================
+ Hits          23881    23884       +3     
+ Misses        22852    22831      -21     
  Partials        327      327              

Files	Coverage Δ
includes/class-sensei-settings.php	57.85% <0.00%> (+0.88%)	⬆️

... and 4 files with indirect coverage changes

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[donnapep]

Message looks good to me!