Deprecate SignalFX support

[izeye]

Now that the SignalFX support from Micrometer has been deprecated in micrometer-metrics/micrometer#5807, the SignalFX support from Spring Boot also needs to be deprecated.

[kiruthiga1793]

Hi @snicoll @izeye I would like to collaborate. Can I work on this issue?

[wilkinsona]

Thanks for the offer, @kiruthiga1793, but I think it's better if a member of the core team takes care of this one. Almost all of the work here is identifying everything that needs to be deprecated. That would have to be done if we make the changes ourselves or if we're reviewing and merging a PR.