[5.1] Replace deprecated substr() with substring()

[C-Lodder]

Summary of Changes

Javascript's substr() method has been deprecated for ages now (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr) and this PR replaces it with substring()

Testing Instructions

1. Inline Help should still work as expected.
2. Add a simple colour field to any XML file (<field name="foo" type="color" label="Test" control="simple" />) and ensure the field works as expected.
3. Ensure toggling the TineMCE presets within the Plugin Manager still works as expected.

I was unable to test the calendar changes as the code has no comments to assist developers, so this might need to be a code review instead.

[ceford]

I have tested this item ✅ successfully on a68b18a

All checked - calendar field in Article edit worked normally.

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[steffans]

I have tested this item ✅ successfully on a68b18a

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[viocassel]

I have tested this item ✅ successfully on a68b18a

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[richard67]

RTC

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[exlemor]

I have not tested this item.

Not testable...

Using Download / Details at the bottom of the Patch page,
https://artifacts.joomla.org/drone/joomla/joomla-cms/5.1-dev/42726/downloads/73221 you receive an error message:

Not Found

The requested URL was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

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[richard67]

@exlemor Possibly the downloads are not available anymore because some time after a PR has been updated last time, the downloads are deleted. As the PR already has 2 good tests I have set it to RTC (ready to commit), so it doesn't need any more tests. But I will trigger a new build of the testing packages now by updating the branch so a bit later new downloads will be available.

[LadySolveig]

Thank you @C-Lodder and also to the testers @ceford @steffans and @viocassel