Bump Erlang/OTP to 27.3 (#120)

OTP 27.2 rejects the new Let's Encrypt "Gen Y" certificate chain served
by hex.pm with a TLS key_usage_mismatch error, due to an over-strict
extended key usage check on intermediate CA certs. This crashed the docs
upload pipeline (Hexpm.get_package/2) on every package release once
hex.pm rotated onto the new chain. The fix shipped in OTP 27.2.2
(public_key OTP-19240, GH-9208); 27.3 is the latest multi-arch base
image tag carrying it. Bump the Debian base date to match.

Author: ericmj

Dockerfile

@@ -1,6 +1,6 @@
 ARG ELIXIR_VERSION=1.18.1
-ARG ERLANG_VERSION=27.2
-ARG DEBIAN_VERSION=bookworm-20241223-slim
+ARG ERLANG_VERSION=27.3
+ARG DEBIAN_VERSION=bookworm-20260518-slim
 
 FROM hexpm/elixir:${ELIXIR_VERSION}-erlang-${ERLANG_VERSION}-debian-${DEBIAN_VERSION} AS build