clang-tidy: Remove a deprecated check

The check performance-faster-string-find has been deprecated, this
replaces it with the new suggested
performance-prefer-single-char-overloads.

Bug: 504772806
Change-Id: I275f9148203642cbf5f252670e6ecf275926bb34
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/401572

.clang-tidy

diff --git a/.clang-tidy b/.clang-tidy
index 3658be4..1529ab8 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -63,7 +63,7 @@
     modernize-use-override,
     modernize-use-transparent-functors,
     modernize-use-uncaught-exceptions,
-    performance-faster-string-find,
+    performance-prefer-single-char-overloads,
     performance-for-range-copy,
     performance-implicit-conversion-in-loop,
     performance-inefficient-algorithm,