io: Retain a defmt-03 feature

Author: chrysn

embedded-io/Cargo.toml

@@ -16,9 +16,14 @@ categories = [
 std = ["alloc"]
 alloc = []
 defmt = ["dep:defmt"]
+defmt-03 = ["defmt", "dep:defmt_03100"]
 
 [dependencies]
 defmt = { version = "1.0.0", optional = true }
+# This feature is deprecated; it is left in place while dependencies such as
+# the worspace's embedded-io-async work with either 0.6 and 0.7, but will be
+# removed going to a 1.0 version.
+defmt_03100 = { package = "defmt", version = "0.3.100", optional = true }
 
 [package.metadata.docs.rs]
 features = ["std"]