Write the brand in lowercase where people see it

The name is inbuxa, lowercase, like the wordmark; INBUXA reads as an
acronym. The admin and webmail already changed. Here that's everything
the server shows people: the brand macro behind the protocol greetings,
the HTTP and SCIM realms, the startup banner and the calendar and contact
PRODID; the first-party OAuth client descriptions; the legacy-protocol
refusals; the default calendar and address book names and the SMTP
greeting default, in the code and the schema served to the admin
(checksum regenerated); startup and shutdown events; the User-Agent;
the sign-in and RSVP pages; the service units; the OpenAPI realm; the
crate descriptions and the README, where it's set in bold.

Identifiers that are uppercase for their own reasons stay: INBUXA_*
settings, SUBSPACE_INBUXA. So do code comments and the AGPL 5(a) notice
lines.

Tests follow: the IMAP ID name, the default collection names, the PRODID
in the iTIP fixtures and the CalDAV free-busy expectations, and the e2e
legacy-protocol refusals. The webdav, imap and jmap suites pass, so do
the unit tests of every crate touched, and 73 of 75 SMTP tests; of the
other two, antispam fails on main too, and queue_retry is a timing flake
that passes on its own.

crates/registry/src/schema/structs_impl.rs

@@ -732,7 +732,7 @@ impl Pickle for AddressBook {
 impl Default for AddressBook {
     fn default() -> Self {
         Self {
-            default_display_name: Some("INBUXA Address Book".to_string()),
+            default_display_name: Some("inbuxa Address Book".to_string()),
             default_href_name: Some("default".to_string()),
             max_v_card_size: 524288u64,
             max_address_books: Some(250u64),
@@ -4597,7 +4597,7 @@ impl Pickle for Calendar {
 impl Default for Calendar {
     fn default() -> Self {
         Self {
-            default_display_name: Some("INBUXA Calendar".to_string()),
+            default_display_name: Some("inbuxa Calendar".to_string()),
             default_href_name: Some("default".to_string()),
             max_attendees: 20u64,
             max_recurrence_expansions: 3000u64,
@@ -4734,7 +4734,7 @@ impl Default for CalendarAlarm {
             allow_external_rcpts: false,
             enable: true,
             from_email: Default::default(),
-            from_name: "INBUXA Calendar".to_string(),
+            from_name: "inbuxa Calendar".to_string(),
             min_trigger_interval: Duration::from_millis(3600000),
             template: Default::default(),
         }
@@ -28310,7 +28310,7 @@ impl MtaStageConnect {
         ExpressionContext {
             expr: &self.smtp_greeting,
             default: Some(Expression {
-                else_: "system('hostname') + ' INBUXA ESMTP at your service'".to_string(),
+                else_: "system('hostname') + ' inbuxa ESMTP at your service'".to_string(),
                 ..Default::default()
             }),
             property: Property::SmtpGreeting,
@@ -28374,7 +28374,7 @@ impl Default for MtaStageConnect {
     fn default() -> Self {
         Self {
             smtp_greeting: Expression {
-                else_: "system('hostname') + ' INBUXA ESMTP at your service'".to_string(),
+                else_: "system('hostname') + ' inbuxa ESMTP at your service'".to_string(),
                 ..Default::default()
             },
             hostname: Expression {