Fix CI deprecation warnings

[ChrisRackauckas]

Summary

• Updated julia-actions/setup-julia from v1 to v2 in Downstream.yml and ReleaseTest.yml
• Updated julia-actions/julia-buildpkg and julia-actions/julia-runtest from v1 to @latest in Downgrade.yml
• Resolves Node.js 16 deprecation warnings by migrating to actions that support Node.js 20

Test plan

• Validated YAML syntax is correct
• CI workflows should run without deprecation warnings
• All existing functionality should continue to work as expected

🤖 Generated with Claude Code

[ChrisRackauckas]

Additional Julia Deprecation Fixes

Added a second commit to fix Julia deprecation references:

• MTKBifurcationKitExt.jl:96 - Updated error message to reference mtkcompile instead of deprecated structural_simplify
• MTKFMIExt.jl:130 - Updated comment to reference mtkcompile instead of deprecated structural_simplify

These changes ensure consistency with the current API and avoid referencing deprecated functions in error messages and comments.

🤖 Generated with Claude Code

[github-actions]

Benchmark Results (Julia vlts)

Time benchmarks

	master	f1e54cf...	master / f1e54cf...
ODEProblem	0.0828 ± 0.0068 s	0.0839 ± 0.0062 s	0.987 ± 0.11
init	0.0614 ± 0.0018 ms	0.0616 ± 0.002 ms	0.996 ± 0.043
large_parameter_init/ODEProblem	0.721 ± 0.011 s	0.713 ± 0.007 s	1.01 ± 0.019
large_parameter_init/init	0.112 ± 0.0022 ms	0.113 ± 0.0028 ms	0.996 ± 0.031
mtkcompile	0.035 ± 0.001 s	0.0351 ± 0.0018 s	0.998 ± 0.06
time_to_load	5.64 ± 0.053 s	5.65 ± 0.039 s	0.998 ± 0.012

Memory benchmarks

	master	f1e54cf...	master / f1e54cf...
ODEProblem	0.588 M allocs: 20 MB	0.588 M allocs: 20 MB	1
init	0.908 k allocs: 0.0479 MB	0.908 k allocs: 0.0479 MB	1
large_parameter_init/ODEProblem	4.64 M allocs: 0.178 GB	4.64 M allocs: 0.179 GB	0.997
large_parameter_init/init	1.75 k allocs: 0.0874 MB	1.75 k allocs: 0.0874 MB	1
mtkcompile	0.233 M allocs: 8.64 MB	0.233 M allocs: 8.64 MB	1
time_to_load	0.153 k allocs: 14.5 kB	0.153 k allocs: 14.5 kB	1

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	f1e54cf...	master / f1e54cf...
ODEProblem	0.0818 ± 0.0031 s	0.0817 ± 0.0042 s	1 ± 0.064
init	0.0561 ± 0.012 ms	0.0573 ± 0.012 ms	0.979 ± 0.3
large_parameter_init/ODEProblem	0.697 ± 0.03 s	0.704 ± 0.0085 s	0.99 ± 0.044
large_parameter_init/init	0.0994 ± 0.011 ms	0.0997 ± 0.012 ms	0.997 ± 0.16
mtkcompile	0.0337 ± 0.0004 s	0.0334 ± 0.00044 s	1.01 ± 0.018
time_to_load	5.61 ± 0.014 s	5.62 ± 0.012 s	0.997 ± 0.0034

Memory benchmarks

	master	f1e54cf...	master / f1e54cf...
ODEProblem	0.579 M allocs: 18.9 MB	0.579 M allocs: 18.9 MB	1
init	0.815 k allocs: 0.0318 MB	0.815 k allocs: 0.0318 MB	1
large_parameter_init/ODEProblem	4.69 M allocs: 0.169 GB	4.69 M allocs: 0.169 GB	1
large_parameter_init/init	1.66 k allocs: 0.0712 MB	1.66 k allocs: 0.0712 MB	1
mtkcompile	0.233 M allocs: 8.08 MB	0.233 M allocs: 8.08 MB	1
time_to_load	0.159 k allocs: 11.2 kB	0.159 k allocs: 11.2 kB	1