Enforce that all docs have last_review_date frontmatter

[issyl0]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

• Part of Review and prune outdated docs #19214.
• So we don't forget to add it to new docs we write.

[issyl0]

homebrew/docs on  lint-frontmatter-presence [$!?] via 💎 v3.3.7 
❯ git diff
diff --git a/docs/Adding-Software-to-Homebrew.md b/docs/Adding-Software-to-Homebrew.md
index 694c0b24aa..b54c020686 100644
--- a/docs/Adding-Software-to-Homebrew.md
+++ b/docs/Adding-Software-to-Homebrew.md
@@ -1,7 +1,3 @@
----
-last_review_date: "1970-01-01"
----
-
 # Adding Software to Homebrew
 
 Is your favourite software missing from Homebrew? Then you're the perfect person to resolve this problem.

homebrew/docs on  lint-frontmatter-presence [$!?] via 💎 v3.3.7 
❯ bundle exec rake lint
mdl $(git ls-files '*.md' | grep -v 'Manpage.md')
grep -L '^last_review_date:' $(git ls-files '*.md' | grep -v 'Manpage.md') | xargs -I {} echo 'File {} is missing last_review_date frontmatter.'
File Adding-Software-to-Homebrew.md is missing last_review_date frontmatter.

[p-linnane]

Great idea! Thanks!