[5.1] Convert mod_post_installation_messages to service provider

[joomlaweby]

Summary of Changes

Converts the administrator post installation messages module to service provider.

Testing Instructions

1. Make sure you have administrator module mod_post_installation_messages published and have some not hidden post installation messages - click on Reset messages button in Post Installation Messages component
2. You should see module output in administrator area

Actual result BEFORE applying this Pull Request

Module works as expected

Expected result AFTER applying this Pull Request

Module works same way as before applying pull request

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

[sandewt]

I have tested this item ✅ successfully on 6a3a01c

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[viocassel]

I have tested this item ✅ successfully on 6a3a01c

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/42987.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/42987.}

[bembelimen]

Thx