Don't uppercase error messages

Run modernize

See https://pkg.go.dev/golang.org/x/tools/gopls/internal/analysis/modernize

It's mostly interface{} -> any, but also two quite useful applications
of slices.Contains.

crypto: harden API surface

Make it explicit that expiration is not being tested with Decrypt, and
test that the expiration is meaningful with DecryptWithExpiration.

Drop leftover references to gql.sr.ht

model: import from gql.sr.ht