Auto merge of #87662 - FabianWolff:rb-string, r=estebank

Suggest `br` if the unknown string prefix `rb` is found

Currently, for the following code:
```rust
fn main() {
    rb"abc";
}
```
we issue the following suggestion:
```
help: consider inserting whitespace here
  |
2 |     rb "abc";
  |       --
```
With my changes (only in edition 2021, where unknown prefixes became an error), I get:
```
help: use `br` for a raw byte string
  |
2 |     br"abc";
  |     ^^
```

0 files changed, 0 insertions, 0 deletions