Replace deprecated model.NameValidationScheme with explicit UTF8Validation (#2051)

* fix(prometheus): use explicit UTF8 validation instead of deprecated NameValidationScheme

model.NameValidationScheme is deprecated in prometheus/common. Replace its three uses (desc.go, labels.go, push/push.go) with the explicit model.UTF8Validation scheme and drop the now-unneeded //nolint:staticcheck directives. The global already defaults to UTF8Validation, so default validation behavior is unchanged; setups that set it to LegacyValidation no longer have that enforced at construction, and UTF-8 validation is more permissive, so no currently-valid name starts failing.

Signed-off-by: Kemal Akkoyun <kemal.akkoyun@datadoghq.com>

* docs(changelog): add entry for UTF-8 validation change

Signed-off-by: Kemal Akkoyun <kemal.akkoyun@datadoghq.com>

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Signed-off-by: Kemal Akkoyun <kemal.akkoyun@datadoghq.com>