Update project dependencies (#168)

I don't remember when was the last time that I did this.

## Notable things

* First, traitobject has finally been upgraded to 0.1.1, this is going
to remove the warning about "future compatibility" received every time
Cartero is compiled.
* In general, traitobject has to go, because it is a transitive
dependency of formdata, which is a crate that has been officially
archived. I have to look for alternatives to generate the
multipart/form-data body of a request, and then remove formdata, because
of the amount of transitive dependencies it introduces.
* srtemplate changed the name of their error type from SrTemplateError
to just Error. The code has been updated to use the new name.

## License check

This PR also introduces `cargo deny` as a tool to inspect for issues
with dependencies. In the meantime I only check the license
compatibility. If at some point a dependency that is not compatible with
GPL-3.0 is added, it will complain. This is important because Cartero is
statically linked due to how Rust works.

Author: danirod

.github/workflows/check.yaml

@@ -60,6 +60,10 @@ jobs:
         run: xvfb-run meson devenv -C build cargo test --manifest-path=../Cargo.toml
       - name: cargo-clippy
         run: meson devenv -C build cargo clippy --manifest-path=../Cargo.toml
+      - name: cargo-deny
+        run: |
+          cargo install --locked cargo-deny
+          meson devenv -C build cargo deny --manifest-path=../Cargo.toml check licenses
       - name: glib-schema
         run: glib-compile-schemas --dry-run --strict data
       - name: glib-desktop