requirements.txt: Instead of pinning Jinja2, use newer MkDocs

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I737c3097d5be11ea0497934272ae2c01da5d365a
diff --git a/requirements.txt b/requirements.txt
index 28c3b7c..cb0f050 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1 +1 @@
-jinja2==3.0.3
+mkdocs>=1.2