Replace static analysis to linear in `cantilever.jl`

[mvanzulli]

The example is linear, so there is no need to run a non-linear analysis.

[mvanzulli]

Sth like this:

"Return the problem solution"
function solve()
    s = structure()
    # -------------------------------
    # Structural Analysis
    # -------------------------------
    (; NSTEPS) = parameters()
    sa = LinearStaticAnalysis(s; NSTEPS)
    # -------------------------------
    # Numerical solution
    # -------------------------------
    sol = solve!(sa)
end;

should be enough ⭕