Always prefer double quotes for docstrings and triple-quoted srings (#7680)

## Summary

At present, `quote-style` is used universally. However, [PEP
8](https://peps.python.org/pep-0008/) and [PEP
257](https://peps.python.org/pep-0257/) suggest that while either single
or double quotes are acceptable in general (as long as they're
consistent), docstrings and triple-quoted strings should always use
double quotes. In our research, the vast majority of Ruff users that
enable the `flake8-quotes` rules only enable them for inline strings
(i.e., non-triple-quoted strings).

Additionally, many Black forks (like Blue and Pyink) use double quotes
for docstrings and triple-quoted strings.

Our decision for now is to always prefer double quotes for triple-quoted
strings (which should include docstrings). Based on feedback, we may
consider adding additional options (e.g., a `"preserve"` mode, to avoid
changing quotes; or a `"multiline-quote-style"` to override this).

Closes https://github.com/astral-sh/ruff/issues/7615.

## Test Plan

`cargo test`

crates/ruff_python_formatter/tests/snapshots/format@expression__bytes.py.snap

@@ -320,17 +320,17 @@ if True:
     b'This string will not include \
         backslashes or newline characters.'
 
-b'''Multiline
+b"""Multiline
 String \"
-'''
+"""
 
-b'''Multiline
+b"""Multiline
 String \'
-'''
+"""
 
-b'''Multiline
+b"""Multiline
 String ""
-'''
+"""
 
 b'''Multiline
 String """
@@ -346,9 +346,9 @@ String '''
 b"""Multiline
 String '"""
 
-b'''Multiline
+b"""Multiline
 String \"\"\"
-'''
+"""
 
 # String continuation
 

crates/ruff_python_formatter/tests/snapshots/format@expression__string.py.snap

@@ -365,17 +365,17 @@ if True:
     'This string will not include \
         backslashes or newline characters.'
 
-'''Multiline
+"""Multiline
 String \"
-'''
+"""
 
-'''Multiline
+"""Multiline
 String \'
-'''
+"""
 
-'''Multiline
+"""Multiline
 String ""
-'''
+"""
 
 '''Multiline
 String """
@@ -391,9 +391,9 @@ String '''
 """Multiline
 String '"""
 
-'''Multiline
+"""Multiline
 String \"\"\"
-'''
+"""
 
 # String continuation
 
@@ -471,16 +471,16 @@ test_particular = [
 
 # Regression test for https://github.com/astral-sh/ruff/issues/5893
 x = (
-    '''aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'''
-    '''bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'''
+    """aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"""
+    """bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"""
 )
 x = (
-    f'''aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'''
-    f'''bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'''
+    f"""aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"""
+    f"""bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"""
 )
 x = (
-    b'''aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'''
-    b'''bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'''
+    b"""aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"""
+    b"""bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"""
 )
 
 # https://github.com/astral-sh/ruff/issues/7460