Update to Go 1.18

Signed-off-by: Luca Comellini <luca.com@gmail.com>

Author: lucacome

.github/workflows/build-pr.yml

@@ -14,7 +14,7 @@ jobs:
       - name: setup go environment
         uses: actions/setup-go@v1
         with:
-          go-version: '1.17.3'
+          go-version: '1.18.9'
       - name: run unit tests
         run: make test
       - name: run acceptance tests

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
       - name: setup go environment
         uses: actions/setup-go@v1
         with:
-          go-version: '1.17.3'
+          go-version: '1.18.9'
       - name: run unit tests
         run: make test
       - name: run acceptance tests

go.mod

@@ -1,6 +1,6 @@
 module oras.land/oras-go
 
-go 1.17
+go 1.18
 
 require (
 	github.com/containerd/containerd v1.6.12