support julia 1.12

[schlichtanders]

when running testitems in vscode julia extension, this will call TestEnv.jl which runs into an error in case the test/Project.toml has a reference back to the toplevel project.
The current code for julia 1.11 does not work for julia 1.12

ERROR: MethodError: no method matching prune_manifest(::Pkg.Types.Manifest, ::Vector{Base.UUID})
The function `prune_manifest` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  prune_manifest(::Pkg.Types.Manifest, !Matched::Set{Base.UUID})
   @ Pkg ~/.julia/juliaup/julia-1.12.1+0.x64.linux.gnu/share/julia/stdlib/v1.12/Pkg/src/Operations.jl:1214
  prune_manifest(!Matched::Pkg.Types.EnvCache)
   @ Pkg ~/.julia/juliaup/julia-1.12.1+0.x64.linux.gnu/share/julia/stdlib/v1.12/Pkg/src/Operations.jl:1193

Stacktrace:
 [1] activate(pkg::String; allow_reresolve::Bool)
   @ TestItemServer.TestEnv ~/.vscode/extensions/julialang.language-julia-1.156.1/scripts/packages/TestItemControllers/packages/TestEnv/src/julia-1.11/activate_set.jl:40
 [2] activate
   @ ~/.vscode/extensions/julialang.language-julia-1.156.1/scripts/packages/TestItemControllers/packages/TestEnv/src/julia-1.11/activate_set.jl:9 [inlined]
 [3] activate_env_request(params::TestItemServer.TestItemServerProtocol.ActivateEnvParams, state::TestItemServer.TestProcessState{var"#2#3"}, token::TestItemServer.CancellationTokens.CancellationToken)
   @ TestItemServer ~/.vscode/extensions/julialang.language-julia-1.156.1/scripts/packages/TestItemControllers/testprocess/TestItemServer/src/TestItemServer.jl:551
 [4] dispatch_msg(x::TestItemServer.JSONRPC.JSONRPCEndpoint{Base.PipeEndpoint, Base.PipeEndpoint}, msg::TestItemServer.JSONRPC.Request, context::TestItemServer.TestProcessState{var"#2#3"})
   @ TestItemServer ~/.vscode/extensions/julialang.language-julia-1.156.1/scripts/packages/TestItemControllers/packages/JSONRPC/src/typed.jl:114
 [5] (::TestItemServer.var"#serve##2#serve##3"{TestItemServer.TestProcessState{var"#2#3"}, TestItemServer.JSONRPC.JSONRPCEndpoint{Base.PipeEndpoint, Base.PipeEndpoint}, TestItemServer.JSONRPC.Request})()
   @ TestItemServer ~/.vscode/extensions/julialang.language-julia-1.156.1/scripts/packages/TestItemControllers/testprocess/TestItemServer/src/TestItemServer.jl:710

[schlichtanders]

This seems alrady be solved in the current version.
Raised the issue here julia-vscode/TestItemControllers.jl#8