Update docs to explain that tested Julia version is 1.12 (#4928)

Author: navidcy

docs/src/index.md

@@ -27,7 +27,7 @@ julia> Pkg.add("Oceananigans")
     Oceananigans requires Julia 1.9 or later.
 
 !!! info "Tested Julia versions"
-    Oceananigans is currently tested on Julia 1.10.
+    Oceananigans is currently tested on Julia 1.12.
 
 If you're [new to Julia](https://docs.julialang.org/en/v1/manual/getting-started/) and its [wonderful `Pkg` manager](https://docs.julialang.org/en/v1/stdlib/Pkg/), the [Oceananigans wiki](https://github.com/CliMA/Oceananigans.jl/wiki) provides [more detailed installation instructions](https://github.com/CliMA/Oceananigans.jl/wiki/Installation-and-getting-started-with-Oceananigans).