Add vips 8.15 to CI

.github/workflows/lint.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         go-version: ["1.21.x"]
-        vips-version: ["8.14"]
+        vips-version: ["8.15"]
     steps:
       - name: Checkout
         uses: actions/checkout@v3

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         go-version: ["1.21.x", "1.20.x", "1.19.x"]
-        vips-version: ["8.14", "8.13"]
+        vips-version: ["8.15", "8.14", "8.13"]
     steps:
       - name: Checkout
         uses: actions/checkout@v3