[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:

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

[ceford]

I have tested this item :white_check_mark: successfully on c7773b90a0424105b6b7b509c62b30acec4cd47a

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[viocassel]

I have tested this item :white_check_mark: successfully on 9e552c8a13254f0b6f85380c4521f9f0443cf491

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[crimle]

I have tested this item :white_check_mark: successfully on 9e552c8a13254f0b6f85380c4521f9f0443cf491

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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?