Update README.md now that 0.7.0 is no longer in alpha

[saolof]

0.7.0 is now released on crates.io

[denschub]

@saolof while you're on it (so that I don't have to submit a conflicting PR), can you also change

sqlx = { version = "0.7", features = [ "runtime-tokio", "tls-native" ] }

and other mentions of tls-native to say tls-native-tls? That's how the feature flag is called:

sqlx/Cargo.toml

Line 77 in af67b13

tls-native-tls = ["sqlx-core/_tls-native-tls", "sqlx-macros/_tls-native-tls"]

and tls-native doesn't actually exist.