debian/cargo/config: Rename to config.toml

to fix a deprecation-warning:

"…/squeekboard/debian/cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier,
you can symlink `config` to `config.toml`"

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/659>

debian/cargo/config.toml

@@ -0,0 +1,14 @@
+# When modifying this file, consider instead
+# to take advantage of the method that Cargo packagers use
+# to set up all the necessary stuff automatically:
+# https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=907629#30
+
+[source.crates-io]
+registry = 'https://github.com/rust-lang/crates.io-index'
+replace-with = 'vendored-sources'
+
+[source.vendored-sources]
+# TODO: Use this instead of `../vendor`,
+# after Squeekboard can also be built with Debian packages only.
+# directory = '/usr/share/cargo/registry'
+directory = '../vendor'