add "document-start" 1st line to all yaml files

The YAML Specification requires three daashes `---` as marker for the
start of a content block. That means, each YAML file needs to start with
`---` on the first line (or after initial comments).

See-also: https://yaml.org/

.github/workflows/dependabot.yaml

@@ -1,3 +1,4 @@
+---
 version: 2
 updates:
 - package-ecosystem: gomod

.github/workflows/trivy.yaml

@@ -1,3 +1,4 @@
+---
 name: Trivy vulnerability scanner
 on:
   push: