Switch to Go 1.15.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/cross-darwin.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory

.github/workflows/cross-freebsd.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory

.github/workflows/cross-openbsd.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory

.github/workflows/cross-windows.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory

.github/workflows/license.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
 
     - name: Check out code
       uses: actions/checkout@v1

.github/workflows/linux.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory

.github/workflows/linux32.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory

.github/workflows/staticcheck.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
 
     - name: Check out code
       uses: actions/checkout@v1

README.md

@@ -31,7 +31,7 @@ go install tailscale.com/cmd/tailscale{,d}
 ```
 
 We only guarantee to support the latest Go release and any Go beta or
-release candidate builds (currently Go 1.14) in module mode. It might
+release candidate builds (currently Go 1.15) in module mode. It might
 work in earlier Go versions or in GOPATH mode, but we're making no
 effort to keep those working.