Update deprecation warnings (#15867)

# Description
- Use #15770 to
  - improve `get --sensitive` deprecation warning
  - add deprecation warning for `filter`
- refactor `filter` to use `where` as its implementation
- replace usages of `filter` with `where` in `std`

# User-Facing Changes
- `get --sensitive` will raise a warning only once, during parsing
whereas before it was raised during runtime for each usage.
- using `filter` will raise a deprecation warning, once

# Tests + Formatting
No existing test broke or required tweaking. Additional tests covering
this case was added.
- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
N/A

---------

Co-authored-by: Bahex <17417311+Bahex@users.noreply.github.com>

crates/nu-std/std/iter/mod.nu

@@ -18,7 +18,7 @@
 export def find [
     fn: closure # the closure used to perform the search 
 ] {
-    filter {|e| try {do $fn $e} } | try { first }
+    where {|e| try {do $fn $e} } | try { first }
 }
 
 # Returns the index of the first element that matches the predicate or -1 if none
@@ -87,7 +87,7 @@ export def scan [ # -> list<any>
 #
 # This is equivalent to 
 #
-#     $in | each $fn | filter $fn
+#     $in | each $fn | where $fn
 @example "Get the squares of elements that can be squared" {
     [2 5 "4" 7] | iter filter-map {|e| $e ** 2}
 } --result [4, 25, 49]
@@ -101,7 +101,7 @@ export def filter-map [
             null 
         }
     } 
-    | filter {|e|
+    | where {|e|
         $e != null
     }
 }