Add a placeholder to the product issue forms

Add a placeholder value for the last updated input in both the product
submission and product update issue forms. This will encourage the
appropriate timestamp format.

.github/ISSUE_TEMPLATE/product-submission-form.yml

@@ -71,6 +71,7 @@ body:
     attributes:
       label: Last updated
       description: When was the product last updated?
+      placeholder: "2021-12-06"
   - type: textarea
     id: product-notes
     attributes:

.github/ISSUE_TEMPLATE/product-update-form.yml

@@ -69,6 +69,7 @@ body:
     attributes:
       label: Last updated
       description: When was the product last updated?
+      placeholder: "2021-12-06"
   - type: textarea
     id: product-notes
     attributes: