Upgrade "soft" deprecations

[cpu]

In two places (Time::try_from and From::from for DnsName) there are "soft" deprecation comments:

webpki/src/time.rs

Lines 26 to 31 in 6dd4a44

	/// Deprecated. Use `TryFrom::try_from`.
	#[cfg(feature = "std")]
	// Soft deprecation. #[deprecated(note = "Use TryFrom::try_from")]
	pub fn try_from(time: std::time::SystemTime) -> Result<Self, ring::error::Unspecified> {
	core::convert::TryFrom::try_from(time)
	}

webpki/src/subject_name/dns_name.rs

Lines 57 to 64 in 6dd4a44

	/// Requires the `alloc` feature.
	// Deprecated
	#[cfg(feature = "alloc")]
	impl From<DnsNameRef<'_>> for DnsName {
	fn from(dns_name: DnsNameRef) -> Self {
	dns_name.to_owned()
	}
	}

We should consider replacing these with "hard" deprecated attributes so that the the deprecations are warned about. Probably best done as part of the next breaking changeset.

[djc]

I think we can just get the #[deprecated] attributes in now. This doesn't break the API, just adds warnings (which is not considered to be semver-incompatible) -- and don't forget that rustls is by far the largest direct user of webpki anyway.