Ruff: explicitly set legacy 0.15 default lint rules.

Prepare for the Ruff 0.16 upgrade by explicitly configuring the legacy
0.15 default ruleset in standalone .ruff.toml files.

Bug: 575585031
Change-Id: Id0620ac3d649eeb741b991e9deeef682fdd2168b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8606869
Owners-Override: Alex Ovsienko <ovsienko@google.com>
Commit-Queue: Alex Ovsienko <ovsienko@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1722557}
NOKEYCHECK=True
GitOrigin-RevId: 249512a6a01d5dc094e26d96433369d77bfc3c46

.ruff.toml

diff --git a/.ruff.toml b/.ruff.toml
index ad212e2..06ae7d0 100644
--- a/.ruff.toml
+++ b/.ruff.toml
@@ -2,4 +2,8 @@
 indent-width = 4
 
 [format]
-quote-style = "preserve"
\ No newline at end of file
+quote-style = "preserve"
+
+[lint]
+# Enable the legacy Ruff 0.15 default ruleset.
+select = ["E4", "E7", "E9", "F"]