Set 2.15.0 as the minimum Dart version

Author: dshukertjr

.github/workflows/main.yml

@@ -15,7 +15,7 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         node: ['12']
-        sdk: [2.14.0, stable, beta, dev]
+        sdk: [2.15.0, stable, beta, dev]
 
     runs-on: ${{ matrix.os }}
 

pubspec.yaml

@@ -5,7 +5,7 @@ homepage: 'https://supabase.io'
 repository: 'https://github.com/supabase-community/functions-dart'
 
 environment:
-  sdk: '>=2.14.0 <3.0.0'
+  sdk: '>=2.15.0 <3.0.0'
 
 dependencies:
   http: ^0.13.4