Formalize attribute deprecation, restrict removal

[lmolkova]

One of the key takeaways from #551 and similar issues on the tooling is that removing an attribute from semconv results in breaking changes for all languages.

Codegen (jinja based) does not take into account previous versions of the spec and does not generate code for removed attributes.

Given that semconv artifact is used by user applications and by different instrumentation libraries (potentially of different versions), removing an attribute leads to breaking changes that are too annoying even for unstable artifact.

The workaround SIGs implemented is to (manually) preserve removed attributes and annotate them as deprecated which created a lot of unnecessary work (and pain).

Proposal

• Do not remove attributes, deprecate them
• Stable -> deprecated attributes should stay till the next major semconv version (forever 🤞)
• We can decide to remove experimental -> deprecated attributes after X versions/months
• Add automated checks to semconv repo to prevent attribute removal

Nice-to-haves:

• add yaml annotations like deprecated_since (and maybe to_be_removed_in )

[lmolkova]

Work items:

• Add backward compatibility check mode build-tools#271
• [chore] Add backward-compatibility check to ci #764
• Update CONTRIBUTING.md with instructions on how to run compat check and how to deprecate attributes and metrics