docs/src/environments.md: update Julia version

[jcsahnwaldt]

1.8 -> 1.9, 1.8.2 -> 1.9.4 (current version)

[jcsahnwaldt]

I guess the current Julia version occurs in other files as well. It might be nice if it could be automatically updated or inserted by the build process. Is there already a build step that replaces variables in MarkDown or other text files?

[IanButterworth]

The julia version on master is actually 1.11 There's unlikely to be another 1.9 release, but we could backport another PR onto the release-1.10 branch to update 1.10 though

[jcsahnwaldt]

I could update the PR and set the version to 1.11 instead. But a better long-term solution would be to replace the hard-coded version by a variable that gets replaced during the build process.

I had a quick look at how the docs are built. Maybe an @eval-block would work? Is the target version available as a Julia variable during the build? If not, can we add it somewhere, e.g. in docs/make.jl? Then I'd change the PR and add something like @eval SomeModule.targetVersion() instead of the hard-coded version strings.