Upgrade fmt dependency to v10.2.1 (#6869)

* Put fmt into version agnostic subdir

* Add fmt to dependency update script

* Remove manually added fmt

* Squashed 'third_party/fmt/' content from commit e69e5f977

git-subtree-dir: third_party/fmt
git-subtree-split: e69e5f977d458f2650bb346dadf2ad30c5320281

third_party/fmt/support/rtd/theme/layout.html

@@ -0,0 +1,17 @@
+{% extends "basic/layout.html" %}
+
+{% block extrahead %}
+<meta charset="UTF-8">
+<meta http-equiv="refresh" content="1;url=https://fmt.dev/latest/">
+<script type="text/javascript">
+    window.location.href = "https://fmt.dev/latest/"
+</script>
+<title>Page Redirection</title>
+{% endblock %}
+
+{% block document %}
+If you are not redirected automatically, follow the <a href='https://fmt.dev/latest/'>link to the fmt documentation</a>.
+{% endblock %}
+
+{% block footer %}
+{% endblock %}

third_party/fmt/support/rtd/theme/theme.conf

@@ -0,0 +1,2 @@
+[theme]
+inherit = basic