Updated min supported version to go 1.18

* Updated min supported version to go 1.18
* Updated go versions used in tests to 1.18

Author: Neo2308

.github/workflows/go.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v2
       with:
-        go-version: 1.15
+        go-version: 1.18
 
     - name: Test
       run: |

v5/go.mod

@@ -1,6 +1,6 @@
 module github.com/evanphx/json-patch/v5
 
-go 1.12
+go 1.18
 
 require (
 	github.com/jessevdk/go-flags v1.4.0