[ffigen] Bump SDK constraint to 4.0.0 (#517)

diff --git a/pkgs/ffigen/example/c_json/pubspec.yaml b/pkgs/ffigen/example/c_json/pubspec.yaml
index 7ef1ce8..3f9c865 100644
--- a/pkgs/ffigen/example/c_json/pubspec.yaml
+++ b/pkgs/ffigen/example/c_json/pubspec.yaml
@@ -5,7 +5,7 @@
 name: c_json_example
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 
 dependencies:
   ffi: ^2.0.1
diff --git a/pkgs/ffigen/example/ffinative/pubspec.yaml b/pkgs/ffigen/example/ffinative/pubspec.yaml
index 2132d8b..9747ec7 100644
--- a/pkgs/ffigen/example/ffinative/pubspec.yaml
+++ b/pkgs/ffigen/example/ffinative/pubspec.yaml
@@ -5,7 +5,7 @@
 name: ffinative_example
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 
 dependencies:
   ffi: ^2.0.1
diff --git a/pkgs/ffigen/example/libclang-example/pubspec.yaml b/pkgs/ffigen/example/libclang-example/pubspec.yaml
index f477957..ab3b75c 100644
--- a/pkgs/ffigen/example/libclang-example/pubspec.yaml
+++ b/pkgs/ffigen/example/libclang-example/pubspec.yaml
@@ -5,7 +5,7 @@
 name: libclang_example
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 
 dependencies:
   ffi: ^2.0.1
diff --git a/pkgs/ffigen/example/objective_c/pubspec.yaml b/pkgs/ffigen/example/objective_c/pubspec.yaml
index 239fcdc..ab1e395 100644
--- a/pkgs/ffigen/example/objective_c/pubspec.yaml
+++ b/pkgs/ffigen/example/objective_c/pubspec.yaml
@@ -5,7 +5,7 @@
 name: objective_c_example
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 
 dependencies:
   ffi: ^2.0.1
diff --git a/pkgs/ffigen/example/shared_bindings/pubspec.yaml b/pkgs/ffigen/example/shared_bindings/pubspec.yaml
index b89304f..b1b5cfd 100644
--- a/pkgs/ffigen/example/shared_bindings/pubspec.yaml
+++ b/pkgs/ffigen/example/shared_bindings/pubspec.yaml
@@ -5,7 +5,7 @@
 name: shared_bindings
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 
 dependencies:
   ffi: ^2.0.1
diff --git a/pkgs/ffigen/example/simple/pubspec.yaml b/pkgs/ffigen/example/simple/pubspec.yaml
index e1a3e14..2755554 100644
--- a/pkgs/ffigen/example/simple/pubspec.yaml
+++ b/pkgs/ffigen/example/simple/pubspec.yaml
@@ -5,7 +5,7 @@
 name: simple_example
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 
 dependencies:
   ffi: ^2.0.1
diff --git a/pkgs/ffigen/example/swift/pubspec.yaml b/pkgs/ffigen/example/swift/pubspec.yaml
index f088c22..8bd5e71 100644
--- a/pkgs/ffigen/example/swift/pubspec.yaml
+++ b/pkgs/ffigen/example/swift/pubspec.yaml
@@ -5,7 +5,7 @@
 name: swift_example
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 
 dependencies:
   ffi: ^2.0.1
diff --git a/pkgs/ffigen/pubspec.yaml b/pkgs/ffigen/pubspec.yaml
index 5dd77d4..dbb5194 100644
--- a/pkgs/ffigen/pubspec.yaml
+++ b/pkgs/ffigen/pubspec.yaml
@@ -8,7 +8,7 @@
 repository: https://github.com/dart-lang/ffigen
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.17.0 <4.0.0'
 
 dependencies:
   ffi: ^2.0.1