[deps] Bump requirement of Jinja2 to 3.1.6

Also, move the pinning down to requirements.server.txt.
This should resolve a few Dependabot alerts.

Supersedes llvm#44

Author: ldionne

requirements.server.txt

@@ -2,4 +2,5 @@
 .
 psycopg2==2.8
 gunicorn==19.9.0
+Jinja2==3.1.6
 progressbar2

setup.py

@@ -119,7 +119,7 @@
         "aniso8601==1.2.0",
         "Flask==0.12.2",
         "Flask-RESTful==0.3.4",
-        "Jinja2==2.11.3",
+        "Jinja2",
         "MarkupSafe==1.1.1",
         "SQLAlchemy==1.3.24",
         "Werkzeug==0.15.6",