Related to https://github.com/coder/coder/issues/15109.
Running postgres tests used to create a new postgres docker container
every time. I believe the slow down might've been caused by that and was
misattributed to postgres performance.
```
coder@main ~/coder ((0e90ac29))> DB=ci gotestsum --packages="./coderd/idpsync" -- -count=1
✓ coderd/idpsync (1.471s)
DONE 91 tests in 4.766s
```
The experimental functions in `golang.org/x/exp/slices` are now
available in the standard library since Go 1.21.
Reference: https://go.dev/doc/go1.21#slices
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>
