Attempt to fix Rust CI (#8411)

by undoing what appears to have broken it: #8372

Author: aardappel

rust/flatbuffers/Cargo.toml

@@ -17,7 +17,7 @@ std = []
 serialize = ["serde"]
 
 [dependencies]
-bitflags = "2.6.0"
+bitflags = "1.2.1"
 serde = { version = "1.0", optional = true }
 
 [build-dependencies]

rust/flexbuffers/Cargo.toml

@@ -24,5 +24,5 @@ deserialize_human_readable = []
 serde = "1.0.119"
 serde_derive = "1.0.119"
 byteorder = "1.4.2"
-num_enum = "0.7.3"
-bitflags = "2.6.0"
+num_enum = "0.5.1"
+bitflags = "1.2.1"