.github: update from 1.18.0-rc1 to 1.18

Author: dkegel-fastly

.github/workflows/build-macos.yml

@@ -16,7 +16,7 @@ jobs:
         include:
           - go-version: '1.17'
             go-stable: true
-          - go-version: '1.18.0-rc1'
+          - go-version: '1.18'
             go-stable: false
     steps:
       - name: Install Go

.github/workflows/linux.yml

@@ -144,7 +144,7 @@ jobs:
         include:
           - go-version: '1.17'
             go-stable: true
-          - go-version: '1.18.0-rc1'
+          - go-version: '1.18'
             go-stable: false
     steps:
       - name: Checkout

.github/workflows/windows.yml

@@ -16,7 +16,7 @@ jobs:
         include:
           - go-version: '1.17'
             go-stable: true
-          - go-version: '1.18.0-rc1'
+          - go-version: '1.18'
             go-stable: false
     steps:
       - name: Install Go