Adjust checks for update mode

[ernilambar]

Fixes #1286

Summary: In update mode only, two checks are relaxed; new mode behavior is unchanged.

Code

Check	New mode	Update mode
outdated_tested_upto_header	ERROR, severity 7	WARNING, severity 5
plugin_header_nonexistent_domain_path	Same as today (warning, severity 6)	Not run (is_new_mode() gate)

Files: includes/Checker/Checks/Plugin_Repo/Plugin_Readme_Check.php, includes/Checker/Checks/Plugin_Repo/Plugin_Header_Fields_Check.php.

[github-actions]

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Co-authored-by: ernilambar <nilambar@git.wordpress.org>
Co-authored-by: davidperezgar <davidperez@git.wordpress.org>
Co-authored-by: frantorres <frantorres@git.wordpress.org>

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