Require Go 1.15 or later

The library github.com/golang-jwt/jwt/v4 requires the FillByte() method
of *big.Int, so we're raising the minimum Go version to 1.15.

.github/workflows/tests.yml

@@ -55,12 +55,6 @@ jobs:
             test_fuse: true
             install_verb: get
 
-          - job_name: Linux
-            go: 1.14.x
-            os: ubuntu-latest
-            test_fuse: true
-            install_verb: get
-
     name: ${{ matrix.job_name }} Go ${{ matrix.go }}
     runs-on: ${{ matrix.os }}