Use must instead of should for 'or' relations

[MARQAS]

Description of the Change

This PR will change the condition from using should to must for 'or' relations in meta queries.

Closes #2932

How to test the Change

• While adding the query to sort the posts using key, it was taking the key inside the should clause, but now it should be under the must clause
• example:

  'ep_integrate' => true,
  'post_type' => 'event',
  'meta_key' => 'event_start',
  'meta_query' => [
    'relation' =>'OR',
    [
      'key' => 'event_start',
      'value' => date('Ymd'),
      'compare' => '>=',
      'type' => 'NUMERIC',
    ],
    [
      'key' => 'event_end',
      'value' => date('Ymd'),
      'compare' => '>=',
      'type' => 'NUMERIC',
    ],
  ],
  'orderby' => 'meta_value_num',
  'order' => 'ASC',
] 

• Observe that the key is inside the must clause instead of should clause

Changelog Entry

Changed - The clause for meta_query when sorting by key and using 'or' relation

Credits

Props @MARQAS

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

@MARQAS although the code you have here might fix the outlying issue, I'm not sure this will actually fix the problem outlined in #2932 without introducing a side-effect.

The problem seems to be coming from this block of code, where we add a new meta query "block" if the 'meta_key' parameter is set in WP_Query.

The problem outlined in #2932 will only happen when the WP_Query object has a meta_key set AND a meta_query using that same meta field inside an OR condition.

We will also need a new PHP Unit test for this one, so it might be helpful to start with the test and adjust the code so it passes.

[felipeelia]

@MARQAS the code you had before was giving you a false positive: if you switch the first clause to <=, it should return all 3 posts but it returned just 2. In general, every time we have an or in WP it will become a should in Elasticsearch, and that was not happening.
I've pushed a commit changing things a bit and I think we are all set with this one. Do you mind giving it a review and letting me know if you have any questions? Thanks!

[MARQAS]

@felipeelia Yes, It is working as expected. Thank you!