[6.0] Replace GMT with UTC

[laoneo]

Summary of Changes

Unifies the code base to use UTC instead of GMT as we offer only UTC as timezone in the options.

Code review as both timezones should be the same.

Link to documentations

Please select:

• [ ] Documentation link for docs.joomla.org:

• [x] No documentation changes for docs.joomla.org needed

• [x] Pull Request link for manual.joomla.org: https://github.com/joomla/Manual/pull/296

• [ ] No documentation changes for manual.joomla.org needed

[Frostholm7]

OK, test on db9d95d7ceb350cdd2c074e4d0e092f253188ed7. Code review, I haven't found any more of this type of change.

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[HLeithner]

Thanks