Downgrade Elixir version to fix PLT generation

Elixir versions >= 1.15 seem to be unable to find the :prettypr module,
causing errors like the following:

```
Finding applications for dialyxir_erlang-25.3.2.10.plt
Finding modules for dialyxir_erlang-25.3.2.10.plt
Creating dialyxir_erlang-25.3.2.10.plt
:dialyzer.run error: Analysis failed with error:
Looking up modules in dialyxir_erlang-25.3.2.10.plt
{undef,[{prettypr,text,["'undefined'"],[]},
        {cerl_prettypr,lay_ann,2,[{file,"cerl_prettypr.erl"},{line,401}]},
        {cerl_prettypr,format,2,[{file,"cerl_prettypr.erl"},{line,261}]},
        {dialyzer_dataflow,signal_guard_failure,5,
                           [{file,"dialyzer_dataflow.erl"},{line,2478}]},
        {dialyzer_dataflow,bind_guard,5,
                           [{file,"dialyzer_dataflow.erl"},{line,1811}]},
        {dialyzer_dataflow,handle_guard_and,5,
                           [{file,"dialyzer_dataflow.erl"},{line,2254}]},
        {dialyzer_dataflow,bind_guard,3,
                           [{file,"dialyzer_dataflow.erl"},{line,1712}]},
        {dialyzer_dataflow,do_clause,7,
                           [{file,"dialyzer_dataflow.erl"},{line,1268}]}]}
```

See: jeremyjh/dialyxir#478

.github/workflows/static-analysis.yml

@@ -30,7 +30,7 @@ jobs:
     strategy:
       matrix:
         elixir:
-        - "1.16"
+        - "1.14"
         otp:
         - "25.3"