Bump Flask, Jinja and Werkzeug to latest versions

Now that we’ve upgraded itsdangerous to the latest version we are:
- unblocked from upgrading to Flask 2, which requires a recent version
  of itsdangerous
- unblocked from upgrading Jinja and Werkzeug to the latest versions,
  which require Flask 2

This commit just does the version upgrades, breaking changes will be
addressed in subsequent commits.

requirements.txt

@@ -57,7 +57,7 @@ eventlet==0.33.1
     # via gunicorn
 fido2==0.9.3
     # via -r requirements.in
-flask==1.1.2
+flask==2.1.2
     # via
     #   -r requirements.in
     #   flask-login
@@ -89,13 +89,15 @@ humanize==4.1.0
     # via -r requirements.in
 idna==3.3
     # via requests
+importlib-metadata==4.11.4
+    # via flask
 itsdangerous==2.0.1
     # via
     #   -r requirements.in
     #   flask
     #   flask-wtf
     #   notifications-utils
-jinja2==3.0.3
+jinja2==3.1.2
     # via
     #   -r requirements.in
     #   flask
@@ -219,7 +221,7 @@ urllib3==1.26.9
     #   requests
 webencodings==0.5.1
     # via bleach
-werkzeug==2.0.3
+werkzeug==2.1.2
     # via
     #   -r requirements.in
     #   flask
@@ -234,6 +236,8 @@ xlrd==2.0.1
     # via pyexcel-xls
 xlwt==1.3.0
     # via pyexcel-xls
+zipp==3.8.0
+    # via importlib-metadata
 
 # The following packages are considered to be unsafe in a requirements file:
 # setuptools