Drop Go 1.13

github.com/go-openapi/runtime's latest version doesn't work with Go
1.13 due to net/http.

Since Go 1.13 is no longer maintained by the Go team, it would be
better to just drop the version rather than asking the package owner
to support the version.

https://pkg.go.dev/net/http#Header.Values

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

Author: Kazuyoshi Kato

.github/workflows/go.yml

@@ -11,7 +11,7 @@ jobs:
   build:
     strategy:
       matrix:
-        go: ['1.13', '1.14', '1.15', '1.16', '1.17']
+        go: ['1.14', '1.15', '1.16', '1.17']
         os: ['ubuntu-18.04', 'ubuntu-20.04']
       fail-fast: false
     name: ${{ matrix.os }} / Go ${{ matrix.go }}