Indicate minimum version 2.1 for memchr dependency

The 2.0 family of memchr doesn't build in this crate.  This is hard to
see using cargo minimal-versions because tokio depends on memchr 2.2,
which raises the dependency using minimal-versions even when tokio is
disabled.  This is a known issue with cargo-minimal-versions:

taiki-e/cargo-minimal-versions#6

So everything looks fine from the perspective of this crate, but
consumers who use quick_xml without the optional features that enable
the tokio dependency have a minimal version down to 2.0, which breaks.

To reproduce in this crate, I commented out the tokio and async_tokio
lines in Cargo.toml and ran cargo minimal-versions, which produces the
errors, and emulates the conditions an external user might experience.

Cargo.toml

@@ -18,7 +18,7 @@ document-features = { version = "0.2", optional = true }
 encoding_rs = { version = "0.8", optional = true }
 serde = { version = "1.0.100", optional = true }
 tokio = { version = "1.0", optional = true, default-features = false, features = ["io-util"] }
-memchr = "2.0"
+memchr = "^2.1"
 
 [dev-dependencies]
 criterion = "0.4"