[4] Remove unused com_media field "upload_mime_illegal"

[ReLater]

All credits to @Kostelano . I only took over his closed pr #39196

Closes #38280

Summary of Changes

• Remove field upload_mime_illegal from com_media settings.
• Deprecate associated language strings.

Testing Instructions

• Code review shoud be sufficient after several confirmations that this field is not used in core. See comments: [4.2] com_media. config. upload_mime_illegal used? #38280 and [4.3] The "Forbidden MIME Types" parameter is not used #39196
• I you think it's worth it: Change/Add mime entries in com_media configuration in field Forbidden MIME Types.
  • Check if these settings have any influence on file uploads in the Media Manager. This should not be the case.

Actual result BEFORE applying this Pull Request

• Settings ignored on file uploads in media manager.

Expected result AFTER applying this Pull Request

• No change. Settings ignored on file uploads in media manager.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[ChristineWk]

I have tested this item ✅ successfully on ee05408

J 4.2.7-dev

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[viocassel]

I have tested this item ✅ successfully on ee05408

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[richard67]

RTC

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[ReLater]

I don't know what

Documentation link for docs.joomla.org

means. I have adapted https://docs.joomla.org/Help4.x:Media:_Options , but ????

[carlitorweb]

Documentation changed ✔️ (https://docs.joomla.org/Help4.x:Media:_Options)

[obuisard]

Thank you @ReLater for the PR!