Refactor course filtering to use Storyblok language settings

[danielmarv]

Resolves #1450

What changes did you make and why did you make them?

• Removed usage of the FF_DISABLED_COURSES environment variable across the codebase.
• Replaced course filtering logic with a check against storyData.content.languages in Storyblok.
• This ensures that course visibility is controlled via Storyblok rather than hardcoded ENV flags.
• Updated /courses page to match how filtering is already done for conversations and shorts.
• Cleaned up the FeatureFlag namespace by removing getDisabledCourses.
• Deleted the FF_DISABLED_COURSES variable from .env files and any related code references.

✅ The /courses page and related content now accurately reflect language availability based on Storyblok configuration.

Did you run tests? Share screenshot of results:

• Yes

How did you find us? (GitHub, Google search, social media, etc.):

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[danielmarv]

@kyleecodes now the tests are passing