Pin rowan to 0.15.15

Author: ShoyuVanilla

crates/syntax/Cargo.toml

@@ -16,7 +16,7 @@ doctest = false
 cov-mark = "2.0.0-pre.1"
 either.workspace = true
 itertools.workspace = true
-rowan = "0.15.15"
+rowan = "=0.15.15"
 rustc-hash.workspace = true
 indexmap.workspace = true
 smol_str.workspace = true