Prepare for porting to lia

This backward-compatible patch prepares ext-lib for
rocq-prover/rocq#7878

Author: maximedenes

examples/ConsiderDemo.v

@@ -4,7 +4,7 @@ Require Import ExtLib.Tactics.Consider.
 Require Import Coq.Bool.Bool.
 Require Import ExtLib.Data.Nat.
 
-Require Import Coq.omega.Omega.
+Require Import Coq.ZArith.ZArith.
 
 Set Implicit Arguments.
 Set Strict Implicit.

theories/Data/ListFirstnSkipn.v

@@ -1,5 +1,5 @@
 Require Import Coq.Lists.List.
-Require Import Coq.omega.Omega.
+Require Import Coq.ZArith.ZArith.
 
 Lemma firstn_app_L : forall T n (a b : list T),
   n <= length a ->