chore(version)!: Bump version number to indicate breaking changes

The recent "tonic" update introduced some breaking changes, see commit
6db36e2 ("bump(cargo)!: bump tonic-build (cargo upgrade)"). Let's
bump the version number.

Signed-off-by: Quentin Monnet <qmo@qmon.net>

Author: qmonnet

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "gateway_config"
-version = "0.11.0"
+version = "0.12.0"
 edition = "2024"
 license = "Apache-2.0"
 rust-version = "1.87.0"