Remove pinned frames url from metadata

[nplasterer]

Talked with product and it doesn't currently feel like there is any value in leaving a url field in the metadata at all so lets just remove it entirely.

THIS IS A BREAKING GROUP CHANGE

[cameronvoell]

this change looks good. Even in rare cases where a user on an old version tried to update this field on a new group that doesnt have the field, we would not have a fork. 👌 see metadata update eval logic for more details:

libxmtp/xmtp_mls/src/groups/group_permissions.rs

Lines 1023 to 1064 in 3b01e70

	/// Evaluates metadata policies for a given set of changes.
	fn evaluate_metadata_policy<'a, I>(
	&self,
	mut changes: I,
	policies: &HashMap<String, MetadataPolicies>,
	actor: &CommitParticipant,
	) -> bool
	where
	I: Iterator<Item = &'a MetadataFieldChange>,
	{
	changes.all(|change| {
	if let Some(policy) = policies.get(&change.field_name) {
	if !policy.evaluate(actor, change) {
	tracing::info!(
	"Policy for field {} failed for actor {:?} and change {:?}",
	change.field_name,
	actor,
	change
	);
	return false;
	}
	return true;
	}
	// Policy is not found for metadata change, let's check if the new field contains the super_admin prefix
	// and evaluate accordingly
	let policy_for_unrecognized_field =
	if change.field_name.starts_with(SUPER_ADMIN_METADATA_PREFIX) {
	MetadataPolicies::allow_if_actor_super_admin()
	} else {
	// Otherwise we default to admin only for fields with missing policies
	MetadataPolicies::allow_if_actor_admin()
	};
	if !policy_for_unrecognized_field.evaluate(actor, change) {
	tracing::info!(
	"Metadata field update with unknown policy was denied: {}",
	change.field_name
	);
	return false;
	}
	true
	})
	}