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.

tests/src/system/tenant.rs

@@ -908,7 +908,7 @@ pub async fn tenant_compat() {
     .expect("expected JSON");
     assert!(
         std::env::var("NO_INSERT").is_ok(),
-        "NO_INSERT must be set, or the copy of INBUXA's data is wiped"
+        "NO_INSERT must be set, or the copy of inbuxa's data is wiped"
     );
 
     let test = TestServerBuilder::new("tenant_compat")