[5.1] Convert mod_feed to service provider

[heelc29]

Summary of Changes

Converts the feed site module to service provider

Testing Instructions

• create site module feed
• check site output

Note: system test added

Actual result BEFORE applying this Pull Request

module works

Expected result AFTER applying this Pull Request

module works

Link to documentations

Please select:

• No documentation changes for docs.joomla.org needed
• No documentation changes for manual.joomla.org needed

[ceford]

I have tested this item ✅ successfully on c7773b9

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[viocassel]

I have tested this item ✅ successfully on 9e552c8

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[crimle]

I have tested this item ✅ successfully on 9e552c8

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[richard67]

RTC

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[bembelimen]

Thank you!

[sandewt]

Still one more comment, see file:
modules/mod_feed/src/Helper/FeedHelper.php

line 65:
deprecated DEPLOY_VERSION will be removed in 7.0

I expected, just like with other recent PRs, eg #42792
deprecated DEPLOY_VERSION will be removed in 6.0

Is there a reason for that?