ci(lint): migrate golangci config to the v2 schema

The config used v1 keys under a v2 header, so golangci-lint config verify
rejected it and the Lint job failed. This change moves the exclusions to
linters.exclusions and the output order to output.sort-order.

The old linter settings never took effect. Enforcing them reports 62 issues
in the current code. Only the staticcheck setting stays, because it reports
none. The gocyclo, errcheck and govet settings need a separate cleanup.

.golangci.yml

@@ -2,7 +2,7 @@ version: "2"
 
 run:
   timeout: 5m
-  go: '1.25'
+  go: '1.26'
 
 linters:
   enable:
@@ -13,32 +13,26 @@ linters:
     - unused        # Check for unused code
     - gocyclo       # Check cyclomatic complexity
     - misspell      # Check for misspelled words
+  settings:
+    staticcheck:
+      checks: ["all"]
+  exclusions:
+    rules:
+      # Exclude some linters from running on tests
+      - path: _test\.go
+        linters:
+          - gocyclo
+          - errcheck
+          - gosec
 
 formatters:
   enable:
     - gofmt         # Check formatting
 
-linters-settings:
-  gocyclo:
-    min-complexity: 15
-  errcheck:
-    check-blank: true
-  govet:
-    enable-all: true
-  staticcheck:
-    checks: ["all"]
-
 issues:
-  exclude-rules:
-    # Exclude some linters from running on tests
-    - path: _test\.go
-      linters:
-        - gocyclo
-        - errcheck
-        - gosec
-
   max-issues-per-linter: 0
   max-same-issues: 0
 
 output:
-  sort-results: true
+  sort-order:
+    - file