Amend style switches (expand -gnatyg). (#1497)

-gnatyg is "check standard GNAT style rules, same as ydISux", but in
GCC 14 it will include a new switch, -gnatyz (check parentheses not
required by operator precedence rules) which will mean that use of
extra parens to clarify intent without deep knowledge of the rules
will trigger fatal warnings; for example in alire-conditional_trees.adb

   function Contains_ORs (This : Tree) return Boolean is
      ((not This.Is_Empty) and then This.Root.Contains_ORs);
       ^

Refer GCC Bugzilla PR 112446.

  * alire_common.gpr (Style_Check_Switches): Remove -gnatyg and replace
    by the pre-GCC 14 equivalent.