Allow proc-macro2 dependency to be flexible (#1102)

Author: alamb

arrow-flight/Cargo.toml

@@ -42,7 +42,7 @@ futures = { version = "0.3", default-features = false, features = ["alloc"]}
 tonic-build = "0.6"
 # Pin specific version of the tonic-build dependencies to avoid auto-generated
 # (and checked in) arrow.flight.protocol.rs from changing
-proc-macro2 = "=1.0.30"
+proc-macro2 = ">1.0.30"
 
 #[lib]
 #name = "flight"