Restore Optim 2 compatibility in optimize!

[orebas]

Summary

Restores optimize! under Optim 2.x while keeping Optim 1.x working.

Changes

• src/optimize.jl — pass method positionally instead of as a keyword. Optim 2 removed the method= form; positional works under both Optim 1 and 2.
• src/GPE.jl — narrow ldiv!(cK::PDMat, x) to x::AbstractVecOrMat. Recent PDMats added a generic ldiv! that was ambiguous with the previous loose form.
• Project.toml — bump Optim compat to "1, 2".

Repro on upstream master

using GaussianProcesses, Random
gp = GPE(rand(2, 20), randn(20), MeanLin(zeros(2)), SE(0.0, 1.0), -3.0)
optimize!(gp)  # MethodError under Optim 2

Test plan

• Reproduced the failure on upstream master under Optim 2.0.1
• All four call patterns from test/optim.jl work end-to-end under Optim 2.0.1 + PDMats 0.11.37
• Backward-compatible with Optim 1.x

Note

A separate Optim 2 issue remains: kwarg shortcuts like optimize!(gp; iterations=50) now need Optim.Options(iterations=50). Not exercised by existing tests; can file a follow-up.