go.mod: update to Go 1.18

Fedora 35 support was dropped, so we can update to a newer Go.

Stable RHEL 8 and 9 and Fedora 36 ships Go 1.18, so let's switch to it.

"//go:build" directives are now apparently enforced by go fmt, so that's why
there were added.

Also, all the github actions were adjusted to use Go 1.18.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

.github/workflows/tests.yml

@@ -66,7 +66,7 @@ jobs:
     - uses: actions/checkout@v3
     - uses: actions/setup-go@v3
       with:
-        go-version: 1.16
+        go-version: 1.18
     - env:
         PGUSER: postgres
         PGPASSWORD: foobar
@@ -106,10 +106,10 @@ jobs:
     name: "⌨ Lint"
     runs-on: ubuntu-latest
     steps:
-      - name: Set up Go 1.16
+      - name: Set up Go 1.18
         uses: actions/setup-go@v3
         with:
-          go-version: 1.16
+          go-version: 1.18
         id: go
 
       - name: Check out code into the Go module directory
@@ -136,10 +136,10 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
 
-      - name: Set up Go 1.16
+      - name: Set up Go 1.18
         uses: actions/setup-go@v3
         with:
-          go-version: 1.16
+          go-version: 1.18
         id: go
 
       - name: Check out code into the Go module directory