[master] Update GitHub Actions (#339)

Copied from `https://github.com/knative/.github/tree/master/workflow-templates`.
/assign n3wscott vagababov
/cc n3wscott vagababov

.github/workflows/knative-go-build.yaml

@@ -27,7 +27,7 @@ jobs:
     name: Build
     strategy:
       matrix:
-        go-version: [1.14.x]
+        go-version: [1.15.x]
         platform: [ubuntu-latest]
 
     runs-on: ${{ matrix.platform }}

.github/workflows/knative-go-test.yaml

@@ -31,7 +31,7 @@ jobs:
     name: Unit Tests
     strategy:
       matrix:
-        go-version: [1.14.x]
+        go-version: [1.15.x]
         platform: [ubuntu-latest]
 
     runs-on: ${{ matrix.platform }}

.github/workflows/knative-style.yaml

@@ -29,10 +29,10 @@ jobs:
 
     steps:
 
-      - name: Set up Go 1.14.x
+      - name: Set up Go 1.15.x
         uses: actions/setup-go@v2
         with:
-          go-version: 1.14.x
+          go-version: 1.15.x
         id: go
 
       - name: Check out code
@@ -83,11 +83,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-
-      - name: Set up Go 1.14.x
+      - name: Set up Go 1.15.x
         uses: actions/setup-go@v2
         with:
-          go-version: 1.14.x
+          go-version: 1.15.x
         id: go
 
       - name: Check out code

.github/workflows/knative-verify.yaml

@@ -27,7 +27,7 @@ jobs:
     name: Verify Deps and Codegen
     strategy:
       matrix:
-        go-version: [1.14.x]
+        go-version: [1.15.x]
         platform: [ubuntu-latest]
 
     runs-on: ${{ matrix.platform }}