Compilation warnings

[miguelmaso]

Afer moving to Julia 1.12 there have been some compilation warnings for a while. Status of some of them:

• num_fields: fixed after upgrading Gridap (see Stable sqrt #130, for example)
• UnionAll fixed by Fixed UnionAll deprecation warning #137
• FESpace: will be fully fixed after the latest release of GridapDistributed (see Added FESpaces qualifier gridap/GridapDistributed.jl#193)
• Gtk4: seems to be an indirect dependency of GridapGmsh