chore: Fix warnings for custom postgres_## cfg flags

[iamjpotts]

Resolves build warnings related to custom cfg conditions that are present in the code because they are set for various CI runs defined in a matrix in the GitHub workflow.

Example warning that disappears with this change:

warning: unexpected `cfg` condition name: `postgres_15`
   --> tests/postgres/types.rs:691:24
    |
691 | #[cfg(any(postgres_14, postgres_15))]
    |                        ^^^^^^^^^^^
    |
    = help: consider using a Cargo feature instead
    = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
             [lints.rust]
             unexpected_cfgs = { level = "warn", check-cfg = ['cfg(postgres_15)'] }
    = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(postgres_15)");` to the top of the `build.rs`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration

Note that even though sqlx supports Postgres 16 and 17, we do not need to add entries for those because they are never referenced as cfg conditions.

Does your PR solve an issue?

No

Is this a breaking change?

No - informs linter of an expected set of possible custom cfg parameter values