Allow OrderedCollections 2 (and Parameters 0.13)

[matthew-liu-1]

Summary

Widens two compat bounds so StanBase can be used in environments that need
OrderedCollections 2, and bumps the patch version to 4.12.5.

-OrderedCollections = "1.4.1"
-Parameters = "0.12"
+OrderedCollections = "1.4.1, 2"
+Parameters = "0.12, 0.13"

Parameters has to move together with OrderedCollections: every Parameters
0.12.x release caps OrderedCollections to 1.x, so 0.13.1 is the only release in
the 0.12–0.13 range that admits OrderedCollections 2. That indirection is why
this looked like a StanSample-only problem at first.

Motivation

@bparbhu — for context on why we're asking for this: we're trying to run
MLJLinearModels at head, which needs OrderedCollections 2 in the resolved
environment. StanBase and StanSample are currently the StanJulia-side pins that
prevent our environment from getting there (together with StanOptimize, which has
the same Parameters = "0.12" cap — companion PR filed there too).

Why no code changes are needed

StanBase's entire surface against these two packages is:

Usage	File	Affected by the 2.0 break?
typeof(input) <: OrderedCollections.OrderedDict	src/common/update_json_files.jl:53	No
@unpack cmdstan_home, exec_path = m	src/common/common_definitions.jl:91	No

OrderedCollections 2.0's breaking changes are: removal of the deprecated
similar(::OrderedDict) / similar(::OrderedSet) / sort(::Dict) methods,
removal of deprecated OrderedSet indexing, convert(OrderedDict, ::AbstractDict)
on an unordered dict now throwing instead of warning, first/last on an empty
collection throwing ArgumentError, and a changed internal OrderedDict struct
layout (new live, firstlive, lastlive fields). StanBase touches none of them.

Parameters 0.13's only breaking change is dropping the deprecated @pack alias
(@pack! stays). StanBase uses @unpack only.

Testing

Julia 1.10.12, Linux x86_64, CmdStan 2.36.0.

• StanBase precompiles cleanly against OrderedCollections 2.0.1 + Parameters 0.13.1.
• Downstream StanSample (with the companion compat PR) runs its full test suite
  green under OrderedCollections 2.0.1: 260 passed, 0 failed.
• Control run: the same suite under OrderedCollections 1.8.2 behaves identically,
  so nothing here is OrderedCollections-2-specific.