[Techassi]

Port of the safe utility function from stackabletech/opa-operator#909.

This PR removes the unsafe and no longer needed UrlExt::join_many function. It additionally also disallows the use of Url::join workspace-wide. This rule can be copied over to operator-templating to enable this rule for all operators as well. The error message renders like this:

error: use of a disallowed method `url::Url::join`
   --> crates/stackable-operator/src/utils/url.rs:165:21
    |
165 |         let _ = url.join("https://evil.example.com/").unwrap();
    |                     ^^^^
    |
    = note: the function resolves its input as a URL, allowing bad actors to craft malicious inputs which replace the URL entirely or traverse the path; use UrlExt::join_safe instead
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.97.0/index.html#disallowed_methods
    = note: requested on the command line with `-D clippy::disallowed-methods`