Show READMEs with .txt extension

What does this MR do?

There are two categories of README files: markup and plain text. For some reason, READMEs with .txt extension disappeared from the plain text category, leaving there only READMEs without extension. This MR returns .txt READMEs into the plain text category so that they are again recognized as README files and rendered on a project page.

Closes #25343 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers