Update lxml requirement from ==4.9.* to ==5.1.* in /server

Updates the requirements on [lxml](https://github.com/lxml/lxml) to permit the latest version.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.9.0...lxml-5.1.0)

---
updated-dependencies:
- dependency-name: lxml
  dependency-type: direct:production
...

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

server/requirements.txt

@@ -14,7 +14,7 @@ django-multifactor==0.6.1
 djangorestframework==3.14.*
 emoji==2.10.*
 icalendar==5.0.*
-lxml==4.9.*
+lxml==5.1.*
 matplotlib==3.7.*
 numpy==1.24.*
 orjson==3.9.*