Provide advance warning of the deprecation and forthcoming removal of OpenTelemetry's ZipkinSpanExporter

[snicoll]

io.opentelemetry.exporter.zipkin.ZipkinSpanExporter has been deprecated as follows:

Zipkin exporter is deprecated in the OpenTelemetry specification (see #4715 ). Expect this artifact to no longer be published in approximately 6 months (mid 2026)

Given our release schedule, we'll have no other choice of removing the auto-configuration in 4.2.x. Perhaps we could gives some heads up in 4.0.x?

[snicoll]

Attention required:

OpenTelemetry's ZipkinSpanExporter has been deprecated and its support will be removed in Spring Boot 4.2.