build: Update serde_yaml to 0.9.4

[Pscheidl]

Description

The test_ser_inline had to be modified according to the new serialization style.

Review Checklist

• Code contains useful comments
• (Integration-)Test cases added (or not applicable)
• Documentation added (or not applicable)
• Changelog updated (or not applicable)
• Cargo.toml only contains references to git tags (not specific commits or branches)

Once the review is done, comment bors r+ (or bors merge) to merge. Further information

[nightkr]

Hi!

We already have a PR for updating to 0.9.x (#450), but we're currently blocked on disabling the new !EnumVariant syntax (which you ran into at https://github.com/stackabletech/operator-rs/pull/455/files#diff-f8e1b80e4d4c689e562ed1f25c1f2fdd39eb590c56e4214ae968d861e9415b6fR239), since that's not really consistent with how Kubernetes understands sum types/enums with values. We'd rather not end up emitting YAML in a format that kubectl isn't able to deserialize correctly.

Is sticking to serde_yaml 0.8.x (for now) blocking a use case for you?

[nightkr]

I'm closing this PR for now since we can't really merge it as is, but the question above is still useful for reprioritizing #450.

[Pscheidl]

Definitely not a blocker. Thanks.