Previously, the base path of a servlet-based management server could be
configured using management.server.servlet.context-path but there was no
equivalent property for WebFlux.

This commit introduces a new property, management.server.base-path,
that can be used with both servlet and reactive management servers. The
existing servlet-specific property has been deprecated in favour of the
new general property. When using the servlet stack, if both the general
property and the servlet-specific property are set, the new general
property takes precedence. When using the reactive stack, only the new
general property is considered.

Closes gh-22906