Drop unsupported OTP matrix entries and bump hexpm fixture to OTP-27.3 / Elixir 1.18.3

ubuntu-22.04 runners do not have prebuilt OTP for 21.3/22.3/23.3 via
setup-beam, so those matrix entries cannot install OTP. Drop them to
match v2.2's matrix lower bound.

Bump HEXPM_OTP and HEXPM_ELIXIR so the cloned hexpm/hexpm main can be
compiled — current main uses Elixir 1.17+ APIs that 1.16.1 cannot run.

Author: ericmj

.github/workflows/main.yml

@@ -54,22 +54,10 @@ jobs:
             elixir: 1.13.4
           - erlang: 24.3
             elixir: 1.12.3
-          - erlang: 23.3
-            elixir: 1.11.3
-          - erlang: 23.3
-            elixir: 1.10.4
-          - erlang: 22.3
-            elixir: 1.9.4
-          - erlang: 21.3
-            elixir: 1.8.1
-          - erlang: 21.3
-            elixir: 1.7.2
-          - erlang: 21.3
-            elixir: 1.6.6
 
     env:
-      HEXPM_OTP: OTP-26.2
-      HEXPM_ELIXIR: v1.16.1
+      HEXPM_OTP: OTP-27.3
+      HEXPM_ELIXIR: v1.18.3
       HEXPM_PATH: hexpm
       HEXPM_ELIXIR_PATH: hexpm_elixir
       HEXPM_OTP_PATH: hexpm_otp