Deprecation warnings due to the deprecation of `u_modified!`

[Kolaru]

On master (and on the latest release) the package often generate deprecation warnings due to the deprecation of u_modified!.

I wanted to open a PR, as it seems rather easy to fix, but I was confused by the following comment

JumpProcesses.jl/src/SSA_stepper.jl

Lines 143 to 145 in bde9c22

	# SciMLBase v3 renamed `u_modified!` to `derivative_discontinuity!` and internal
	# callback code now calls the new name. Define the method when available so
	# callbacks keep working on v3. Guarded so the file still loads on v2.

So I am not sure what should be done there, especially considering that the compat in Project.toml is limited to SciML v3, which seems to contradict this comment to me.

[ChrisRackauckas]

This package should probably rename it derivative_discontinuity in its ssaintegrator to match the standard convention and update those functions to be instead changing that property.