Update MetaGraphsNext requirement to 0.8.0 in /docs (#117)

Updates the requirements on [MetaGraphsNext](https://github.com/JuliaGraphs/MetaGraphsNext.jl) to permit the latest version.
- [Release notes](https://github.com/JuliaGraphs/MetaGraphsNext.jl/releases)
- [Commits](v0.1.0...v0.8.0)

---
updated-dependencies:
- dependency-name: MetaGraphsNext
  dependency-version: 0.8.0
  dependency-type: direct:production
...

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docs/Project.toml

@@ -13,3 +13,4 @@ Documenter = "1"
 Graphs = "1.13.3"
 JLD2 = "0.6"
 MetaGraphs = "0.9.0"
+MetaGraphsNext = "0.8.0"