deps(fluent-fallback): Bump once_cell to 1.21

Author: alerque

fluent-fallback/Cargo.toml

@@ -22,7 +22,7 @@ rustc-hash.workspace = true
 unic-langid.workspace = true
 async-trait = "0.1"
 chunky-vec = "0.1"
-once_cell = "1.19"
+once_cell = "1.21"
 pin-cell = "0.2"
 
 [dev-dependencies]