FlexMailer\Listener\OpenTracker: remove deprecated isLegacy check

[semseysandor]

Overview

Fix Todo: After v5.21 goes EOL, remove the $isLegacy check.

Before

Checking if Civi version is greater than 5.23.alpha.

After

Check removed.

[civibot]

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[semseysandor]

A quick review of the test console output, I think failure is caused by this:

md5_file(/tmp/drush_tmp_1686748201_6489bc2952b3a/drupal-7.98.tar.gz):  [war  
  ning]                                                                        
  Failed to open stream: No such file or directory drush.inc:727               
  File drupal-7.98.tar.gz is corrupt (wrong md5 checksum).                 [e  
  rror]   

Guess it's not the unit-tests?

[totten]

civibot, test this please

[totten]

Looks pretty good to me. Let's see if another test-run clears that up.