.github/workflows: drop 1.15 from long test

Change-Id: If08c53689f443541fff6af4e475a04c29f3efd04
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/418540
Reviewed-by: Suzy Mueller <[email protected]>

Author: hyangah

.github/workflows/test-long-all.yml

@@ -17,7 +17,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
         version: ['stable', 'insiders']
-        go: ['1.15', '1.16', '1.17', '1.18']
+        go: ['1.16', '1.17', '1.18']
 
     steps:
       - name: Clone repository

.github/workflows/test-long.yml

@@ -16,7 +16,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, windows-latest] # TODO: reenable macos-latest
         version: ['stable']
-        go: ['1.15', '1.16', '1.17', '1.18']
+        go: ['1.16', '1.17', '1.18']
 
     steps:
       - name: Clone repository