Split out incubating artifact

[jack-berg]

Resolves #22. Embodies this proposal. Uses some of the concepts in #40, but without breaking out classes for each top level namespace.

Summary:

• Break out new io.opentelemetry.semconv:opentelemetry-semconv-incubating artifact for experimental semantic conventions.
• Incubating artifact has files IncubatingResourceAttributes and IncubatingSemanticAttributes. (I think breaking out classes for top level namespaces in #40 is a good thing to do but can be done separately.)
• Delete all hardcoded deprecated constants from jinja template. We've been keeping those around while we work out a strategy for #22, and we now have a strategy. Additionally, its not obvious where we would put them. They don't belong in the artifact for stable conventions, and the names of the classes changes in the incubating artifact, so would break users anyway. Furthermore, I think we should break out classes for top level namespaces in a followup PR, which makes it harder to justify including the old deprecated constants.
• Add new SchemaUrls class with constants of form public static final String V1_23_1 = "https://opentelemetry.io/schemas/1.23.1". This is a running list of schema URL constants which needs to be manually updated.

[jack-berg]

I pushed a commit that allows the same thing to be accomplished without https://github.com/open-telemetry/build-tools/pull/243.

Its less convenient, but allows this PR to proceed independently of #40. Marking as ready to review.