add go.mod and update CI

Author: kevinburke

.github/workflows/ci.yml

@@ -2,16 +2,12 @@ on: [push, pull_request]
 name: Test
 jobs:
   test:
-    strategy:
-      matrix:
-        go-version: [1.15.x]
-        platform: [ubuntu-18.04]
-    runs-on: ${{ matrix.platform }}
+    runs-on: ubuntu-18.04
     steps:
     - name: Install Go
       uses: actions/setup-go@v2
       with:
-        go-version: ${{ matrix.go-version }}
+        go-version: 1.23.x
     - uses: actions/checkout@v2
       with:
          path: './src/github.com/kevinburke/tss'

go.mod

@@ -0,0 +1,3 @@
+module github.com/kevinburke/tss
+
+go 1.20