Reduce indexmap version to 1.6.2 (#1501)

deno_core currently uses `=1.6.2` for indexmap, and Cargo refuses to resolve a version with sqlx's requirement of 1.7.0.

https://github.com/denoland/deno/blob/004d07dccd69c9fae8370665632d90401f770938/core/Cargo.toml#L18

Author: dimfeld

sqlx-core/Cargo.toml

@@ -163,4 +163,4 @@ stringprep = "0.1.2"
 bstr = { version = "0.2.14", default-features = false, features = ["std"], optional = true }
 git2 = { version = "0.13.20", default-features = false, optional = true }
 hashlink = "0.7.0"
-indexmap = "1.7.0"
+indexmap = "1.6.2"