Update diesel-async to 0.6, and remove an unnecessary Sync bound

[the10thWiz]

The Sync bound on Pool is, as far as I can tell, unneeded. diesel-async's mysql connection pool isn't Sync anymore,
which is why bound needed to be removed.

closes: #2969

[mcclure]

I can successfully complete the tests now on my machine. I think this PR works

[mcclure]

By the way, in my testing against this PR, you could bump diesel-async as far as 0.6.1 and diesel to 2.2.12.

diff --git a/contrib/db_pools/lib/Cargo.toml b/contrib/db_pools/lib/Cargo.toml
index a0a911bd..b88f52db 100644
--- a/contrib/db_pools/lib/Cargo.toml
+++ b/contrib/db_pools/lib/Cargo.toml
@@ -65,13 +65,13 @@ features = ["compat-3-0-0", "rustls-tls"]
 optional = true
 
 [dependencies.diesel-async]
-version = "0.6.0"
+version = "0.6.1"
 default-features = false
 features = ["async-connection-wrapper"]
 optional = true
 
 [dependencies.diesel]
-version = "2.1"
+version = "2.2.12"
 default-features = false
 optional = true

(There is a diesel 2.3.x but diesel_async is not yet ready to support it.)

With this patch both ./scripts/test.sh --contrib and my diesel_sqlite test application work. If this PR merges as is, I will probably submit the above patch as an additional PR.