Remove prometheus legacy validation warning (#6590)

I have been chatting with @bwplotka, who wouldn't recommend removing the
support for the legacy scheme now.

Indeed, some system don't support the new scheme yet (such as cortex).
So removing this will entirely prevent those users form upgrading their
SDK.

They are going to remove the deprecation from the legacy method on
common, at least until the new scheme can be used in enough systems.

---------

Co-authored-by: Sam Xie <sam@samxie.me>

CHANGELOG.md

@@ -8,14 +8,6 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
-> [!WARNING]
-> This is the last version to support `model.LegacyValidation` for
-> `go.opentelemetry.io/otel/exporters/prometheus`.
-> The next version (v0.59.0) will only support the default `model.UTF8Validation`.
->
-> See also [Change default validation scheme to UTF8Validation](https://github.com/prometheus/common/pull/724)
-> in the prometheus repository.
-
 ### Added
 
 - Add exponential histogram support in `go.opentelemetry.io/otel/exporters/prometheus`. (#6421)

exporters/prometheus/config.go

@@ -29,7 +29,7 @@ type config struct {
 
 var logDeprecatedLegacyScheme = sync.OnceFunc(func() {
 	global.Warn(
-		"prometheus exporter legacy scheme deprecated: support for the legacy NameValidationScheme will be removed in the next release",
+		"prometheus exporter legacy scheme deprecated: support for the legacy NameValidationScheme will be removed in a future release",
 	)
 })