Updated .golangci.toml to exclude specific staticcheck rule

- Added exclusion rule for `staticcheck` linters in `internal/server/sendmail.go`
- Ignored false positive `QF1012` related to strings.Builder usage

Signed-off-by: Winni Neessen <wn@neessen.dev>

Author: wneessen

.golangci.toml

@@ -46,3 +46,11 @@ paths = [
   'builtin$',
   'examples$'
 ]
+
+## False positive, strings.Builder cannot be used as io.Writer for fmt.Fprintf
+[[linters.exclusions.rules]]
+linters = [
+    'staticcheck'
+]
+path = 'internal/server/sendmail.go'
+text = 'QF1012:'