Improve D-Bus byte-array conversion

Improve D-Bus byte-array conversion

* doc/misc/dbus.texi (Type Conversion): Adapt dbus-byte-array-to-string.

* etc/NEWS: D-Bus byte array conversion works over raw UTF-8 bytes.
Fix typos.

* lisp/net/dbus.el (dbus-string-to-byte-array)
(dbus-byte-array-to-string): BYTE-ARRAY must be an UTF-8 raw bytes
sequence.  Make optional argument MULTIBYTE obsolete.  (Bug#70301)
(dbus-call-method-handler, dbus-register-signal)
(dbus-escape-as-identifier): Use `length=' and `length>'.

* test/lisp/net/dbus-tests.el (dbus--test-method-handler)
(dbus-test09-get-managed-objects): Use `length='.
(dbus-test01-type-conversion): Extend test.

* test/lisp/net/secrets-tests.el (secrets-test03-items): Extend test.