Framework for deprecating pipeline functions; and deprecate `remove_field`

[patrickmann]

Follow-up issue related to #19259, #19268

What?

• A way to deprecate functions including making users aware of this in the UI.
• Deprecate remove_field with the target of full removal in 7.0.
• Add a rate-limited logger to RemoveField, signaling the deprecation.

Why?

Pipeline functions are like the GL API - we cannot easily introduce breaking changes or deprecate them, because it may cause existing functions to behave in unexpected ways; potentially even leading to data loss. Breaking changes need to be communicated well ahead of time, in a way that is visible to users.

Case in point: #19259