remove --not flag for 'str contains' (nushell#12837)

# Description
This PR resolves an inconsistency between different `str` subcommands,
notably `str contains`, `str starts-with` and `str ends-with`. Only the
`str contains` command has the `--not` flag and a desicion was made in
this nushell#12781 PR to remove the `--not` flag and use the `not` operator
instead.

Before:
`"blob" | str contains --not o`
After:
`not ("blob" | str contains o)` OR `"blob" | str contains o | not $in`

> Note, you can currently do all three, but the first will be broken
after this PR is merged.

# User-Facing Changes
- remove `--not(-n)` flag from `str contains` command
  - This is a breaking change!

# Tests + Formatting
- [x] Added tests
- [x] Ran `cargo fmt --all`
- [x] Ran `cargo clippy --workspace -- -D warnings -D
clippy::unwrap_used`
- [x] Ran `cargo test --workspace`
- [ ] Ran `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"`
    - I was unable to get this working.
```
Error: nu::parser::export_not_found

  × Export not found.
   ╭─[source:1:9]
 1 │ use std testing; testing run-tests --path crates/nu-std
   ·         ───┬───
   ·            ╰── could not find imports
   ╰────
```
^ I still can't figure out how to make this work 😂 

# After Submitting
Requires update of documentation

crates/nu-command/src/strings/str_/contains.rs

@@ -40,7 +40,7 @@ impl Command for SubCommand {
                 "For a data structure input, check strings at the given cell paths, and replace with result.",
             )
             .switch("ignore-case", "search is case insensitive", Some('i'))
-            .switch("not", "does not contain", Some('n'))
+            .switch("not", "DEPRECATED OPTION: does not contain", Some('n'))
             .category(Category::Strings)
     }
 
@@ -59,6 +59,20 @@ impl Command for SubCommand {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        if call.has_flag(engine_state, stack, "not")? {
+            nu_protocol::report_error_new(
+                engine_state,
+                &ShellError::GenericError {
+                    error: "Deprecated option".into(),
+                    msg: "`str contains --not {string}` is deprecated and will be removed in 0.95."
+                        .into(),
+                    span: Some(call.head),
+                    help: Some("Please use the `not` operator instead.".into()),
+                    inner: vec![],
+                },
+            );
+        }
+
         let cell_paths: Vec<CellPath> = call.rest(engine_state, stack, 1)?;
         let cell_paths = (!cell_paths.is_empty()).then_some(cell_paths);
         let args = Arguments {
@@ -120,15 +134,6 @@ impl Command for SubCommand {
                     Value::test_bool(false),
                 ])),
             },
-            Example {
-                description: "Check if list does not contain string",
-                example: "[one two three] | str contains --not o",
-                result: Some(Value::test_list(vec![
-                    Value::test_bool(false),
-                    Value::test_bool(false),
-                    Value::test_bool(true),
-                ])),
-            },
         ]
     }
 }