fix: replace dropRightWhile with dropEndWhile

[Neptunium931]

Hello, thank you for all the work that has allowed me to discover Lean, but it seems that there is a use of a deprecated function dropRightWhile

def main : IO Unit := do
  let stdin ← IO.getStdin
  let stdout ← IO.getStdout

  stdout.putStrLn "How would you like to be addressed?"
  let input ← stdin.getLine
  let name := input.dropRightWhile Char.isWhitespace

  stdout.putStrLn s!"Hello, {name}!"

lean --run hello.lean
hello.lean:7:20: warning: `String.dropRightWhile` has been deprecated: Use `String.dropEndWhile` instead

Note: The updated constant has a different type:
  {ρ : Type} → String → (pat : ρ) → [String.Slice.Pattern.BackwardPattern pat] → String.Slice
instead of
  String → (Char → Bool) → String

I used the following command to make the modification; I think I've replaced it in the only necessary place.

find . -type f -iname *.lean -exec sed -i 's/dropRightWhile/dropEndWhile/g' {} +

lean version :

$ lean --version
Lean (version 4.29.0-pre, commit 6cf632bef2d64028df39e3cb790deb9e6e431067, Release)