build(deps): update lightning requirement in /requirements

Updates the requirements on [lightning](https://github.com/Lightning-AI/lightning) to permit the latest version.
- [Release notes](https://github.com/Lightning-AI/lightning/releases)
- [Commits](Lightning-AI/pytorch-lightning@1.8.0...2.4.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements/_docs.txt

@@ -11,7 +11,7 @@ sphinx-togglebutton ==0.3.2
 sphinx-copybutton ==0.5.2
 sphinx-gallery ==0.17.1
 
-lightning >=1.8.0, <2.4.0
+lightning >=1.8.0, <2.5.0
 lightning-utilities ==0.11.6
 pydantic > 1.0.0, < 3.0.0