Consider removing support for P-521

[sleevi]

A number of user agents intentionally decline to implement P-521, due to the significant performance overheads incurred relative to the limited security benefit provided in practice. For example, both Chrome and Firefox prohibit such certificates within TLS connections.

The current language includes P-521 as optional, as captured at

openscreenprotocol/index.bs

Lines 363 to 371 in 5488c7b

	The [=agent certificate=] must have the following characteristics:
	* 256-bit, 384-bit, or 521-bit ECDSA public key
	* Self-signed
	* Supporting the at least one of the following signature algorithms:
	* secp256r1_sha256
	* secp384r1_sha384
	* secp521r1_sha512
	* Valid for signing

This will likely exacerbate interoperability issues, and also arguably incompatible with the goal stated in Non-Functional Requirements, namely

openscreenprotocol/index.bs

Lines 206 to 210 in 5488c7b

	1. It should be possible to implement an OSP agent using modest
	hardware requirements, similar to what is found in a low end smartphone,
	smart TV or streaming device. See the [Device
	Specifications](https://w3c.github.io/openscreenprotocol/device_specs.html)
	document for agent hardware specifications.

Suggestion: Remove P-521.

[mfoltzgoogle]

Closed by #295.