[6.0] Introduce Joomla 6.0 compatibility plugin

[HLeithner]

In the today's maintainer meeting we came to the conclusion that making a new plugin with a new name makes it easier
for us to update and maintain in the future.

Considerations:

• We need to activate the b/c plugin before upgrading 5.4 to 6.0 (it's not possible in the upgrade process)
• We would like to force the user to disable the b/c plugin in 5.4 before upgrading (if it site crashes in 5.4 it could be easily recovered by reactivating the plugin, but if it crashes after uninstall in the 6.0 upgrade process there is no way back)
• Both checks can be done in the pre update checker but only if we have 2 separated plugins

Summary of Changes

• Create the compat6 plugin
• Removed the compat plugin
• Moved already merged changes from old to compat6 plugin
• Removed all b/c functionally for 5 with the old compat plugin as discussed in the maintainers meeting from February

Testing Instructions

• New install and test with and without extensions
• Update from the latest 5.4 nightly build to the patched package or custom update URL created by Drone for this PR, having an extension installed on the 5.4 which will require the compat6 plugin to be enabled on Joomla 6.

Actual result BEFORE applying this Pull Request

Everything works without the old b/c plugin

After updating from 5.4, the old b/c plugin is still there.

Expected result AFTER applying this Pull Request

Everything works with the new b/c plugin

After updating from 5.4, the old b/c plugin is removed.

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

[LadySolveig]

I have tested this item ✅ successfully on d8cbde2

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[richard67]

I have tested this item ✅ successfully on d8cbde2

Tested update from 5.4 as well as new installation.

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[richard67]

RTC

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[softforge]

Thank you @HLeithner and @richard67 and all who worked on it to make it happen.