[6.0] README.md delete heavy development note

[muhme]

Summary of Changes

Clean heavy development note in README.md as we have created release candidate.

Testing Instructions

Open README.md

Actual result BEFORE applying this Pull Request

README.md starts with heavy development note.

Expected result AFTER applying this Pull Request

No heavy development note anymore.

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

[tecpromotion]

RTC

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[brianteeman]

The same change needs to be made to the readme.txt

[muhme]

Also cleaned "heavy development" note in README.txt and removed RTC label

[brianteeman]

I have tested this item ✅ successfully on a250913

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