Update flask-migrate requirement from ~=2.7.0 to ~=3.1.0

Updates the requirements on [flask-migrate](https://github.com/miguelgrinberg/flask-migrate) to permit the latest version.
- [Release notes](https://github.com/miguelgrinberg/flask-migrate/releases)
- [Changelog](https://github.com/miguelgrinberg/Flask-Migrate/blob/main/CHANGES.md)
- [Commits](miguelgrinberg/Flask-Migrate@v2.7.0...v3.1.0)

---
updated-dependencies:
- dependency-name: flask-migrate
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements-freeze.txt

@@ -17,7 +17,7 @@ dnspython==2.2.1
 Flask==1.1.4
 Flask-gzip==0.2
 Flask-Mail==0.9.1
-Flask-Migrate==2.7.0
+Flask-Migrate==3.1.0
 Flask-pyoidc==3.7.0
 Flask-SQLAlchemy==2.5.1
 future==0.18.2

requirements.txt

@@ -1,6 +1,6 @@
 Flask-Gzip==0.2
 Flask-Mail==0.9.1
-Flask-Migrate~=2.7.0
+Flask-Migrate~=3.1.0
 Flask-pyoidc~=3.7.0
 Flask~=1.1.4
 csh_ldap~=2.3.1