Rename into decimal to into float (#9979)

# Description
We keep "into decimal" for a release and warn through a message that it
will be removed in 0.86.

All tests are updated to use `into float`

# User-Facing Changes
`into decimal` raises a deprecation warning, will be removed soon.
Use `into float` as the new functionally identical command instead.

```
~/nushell> 2 | into decimal
Error:   × Deprecated command
   ╭─[entry #1:1:1]
 1 │ 2 | into decimal
   ·     ──────┬─────
   ·           ╰── `into decimal` is deprecated and will be removed in 0.86.
   ╰────
  help: Use `into float` instead


2
```

# Tests + Formatting
Updated

---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

crates/nu-command/tests/commands/str_/mod.rs

@@ -140,7 +140,7 @@ fn converts_to_decimal() {
         r#"
             echo "3.1, 0.0415"
             | split row ","
-            | into decimal
+            | into float
             | math sum
         "#
     ));