Error if `version` isn't a string

[IanButterworth]

Related: #203

[goerz]

That would be great, at least when that makes the version ambiguous, as in 1.10 -> "1.1", see JuliaDocs/Documenter.jl#2474

[IanButterworth]

I looked into this a little and didn't find a way to query what the type of the entry in the yml file is. And while it's possible for workflow_dispatch inputs to specify a type, it doesn't seem possible for general inputs

[IanButterworth]

Some fruitless exploration in #225 and filed an issue upstream actions/toolkit#1682