fix(sync-translated-content): preserve l10n frontmatter

[caugner]

Summary

Fixes an issue with sync-translated-content encountered in mdn/translated-content#21044 (comment).

Problem

Our sync-translated-content tool removes the l10n frontmatter key.

Solution

Add l10n to the list of optional front matter keys.

---

How did you test this change?

• Checked out mdn/content @ b68d6456477c19b1fed8fc6bc99eff8972b1af29
• Checked out mdn/translated-content @ f5db1f7163d985ff55b90d11f1efe2fc34ad3445
• Ran yarn tool sync-translated-content ko
• Verified the diff of files/ko/glossary/gzip_compression/index.md:

Before

diff --git a/files/ko/glossary/gzip_compression/index.md b/files/ko/glossary/gzip_compression/index.md
index 2662038da6..087779623b 100644
--- a/files/ko/glossary/gzip_compression/index.md
+++ b/files/ko/glossary/gzip_compression/index.md
@@ -1,8 +1,6 @@
 ---
 title: GZip 압축 (GZip compression)
-slug: Glossary/GZip_compression
-l10n:
-  sourceCommit: ada5fa5ef15eadd44b549ecf906423b4a2092f34
+slug: Glossary/gzip_compression
 ---
 
 {{GlossarySidebar}}

After

diff --git a/files/ko/glossary/gzip_compression/index.md b/files/ko/glossary/gzip_compression/index.md
index 2662038da6..0e12e51c7a 100644
--- a/files/ko/glossary/gzip_compression/index.md
+++ b/files/ko/glossary/gzip_compression/index.md
@@ -1,6 +1,6 @@
 ---
 title: GZip 압축 (GZip compression)
-slug: Glossary/GZip_compression
+slug: Glossary/gzip_compression
 l10n:
   sourceCommit: ada5fa5ef15eadd44b549ecf906423b4a2092f34
 ---