Deprecate PushRegistryConfig#numThreads

[snicoll]

The PushRegistryConfig#numThreads javadoc states:

• @deprecated since 1.1.13 because this configuration is not used

We're using Micrometer 1.3 so that deprecation is quite overdue. We can deprecate it in 2.2.x and remove it in 2.3 still.

[snicoll]

The version of micrometer we have does not flag the property has deprecated yet as it's quite recent, see micrometer-metrics/micrometer#1900