[chore] add remove-toolchain make cmd

[mowies]

Description:

This PR adds a new remove-toolchain make command which removes the toolchain directive from all go.mod files.
The new make command is also added to the update-otel make target.

Link to tracking Issue:

• fixes make update-otel shouldn't update the toolchain #33600
• could potentially also work for [CI] Avoid toolchain updates on make gotidy runs #35160

Testing:

• add a toolchain go1.22.5 directive to any of the go.mod files in this repo
• call make remove-toolchain
• check if the toolchain directive was removed

[mowies]

left this in since this file will be removed with #34844 anyways and this also satisfies the pipeline

[songy23]

👍

[mx-psi]

I would expect this PR to also remove the toolchain directive from e.g.

opentelemetry-collector-contrib/cmd/otelcontribcol/go.mod

Line 7 in 313f4d0

toolchain go1.22.7

[mowies]

@mx-psi that would usually happen but otelcontribcol is specifically excluded from the go modules here.
Reasoning for that was given here

[mx-psi]

Thanks for explaining!