[deps] Update serde_yaml to 0.9

[aborg-dev]

At the moment we depend on serde_yaml version "0.8.16". This version is known to have a few bugs when parsing invalid YAML files that were triggered by #8310:

• Parsing empty file returns an error instead of empty YAML map
• Parsing a map with empty key does not return an error and instead yields a map with empty string being a key

Both of these bugs are non-critical for real usage and could be fixed with additional checks, but this complicates the code. This issue tracks the need to update the version of serde_yaml and remove these workarounds (that will be marked with TODOs in the code).

At the moment updating serde_yaml to 0.9 is blocked on two crates:

• paperclip: I've updated it to latest 0.7.1 version that depends on serde_yaml 0.9 according to Cargo.toml, but on Cargo it still depends on the serde_yaml 0.8. This should be a matter of raising an issue in the project to update a package in Cargo.
• insta: we need at least "1.18.0", but updating to this version results in many other version conflicts in dependencies and breaks the build due to changed interface, so this requires some migration work

[aborg-dev]

The serde_yaml is update and TODOs in the code are fixed now, so closing this as done.