Bump nbconvert from 6.3.0 to 6.5.1 in /examples/research_projects/lxmert (#18742)

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 6.3.0 to 6.5.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](https://github.com/jupyter/nbconvert/compare/6.3.0...6.5.1)

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

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examples/research_projects/lxmert/requirements.txt

@@ -43,7 +43,7 @@ matplotlib==3.3.1
 mistune==2.0.3
 msgpack==0.6.2
 nbclient==0.5.0
-nbconvert==6.3.0
+nbconvert==6.5.1
 nbformat==5.0.7
 nest-asyncio==1.4.0
 notebook==6.4.12