Fix crash when report_bug_threshold=array in access_has_bug_level

When displaying a bug for which the user is not the reporter,
$g_limit_reporters=ON and the workflow is set so report_bug_threshold is
an array, MantisBT crashes with "PHP Fatal error: Unsupported operand
types".

This is due to use of '+ 1' to indicate that user should have the next
higher access level to view the issue. We now use the same logic but
within a foreach loop to check against each array element.

Fixes #15538