Enable feature maybe_expr in the compiler for Erlang/OTP 26

Documentation:
https://www.erlang.org/doc/system/expressions.html#maybe
https://www.erlang.org/eeps/eep-0049
https://www.erlang.org/eeps/eep-0060

mix.exs

@@ -99,6 +99,7 @@ defmodule Ejabberd.MixProject do
              if_version_below(~c"25", [{:d, :OTP_BELOW_25}]) ++
              if_version_below(~c"26", [{:d, :OTP_BELOW_26}]) ++
              if_version_below(~c"27", [{:d, :OTP_BELOW_27}]) ++
+             if_version_below(~c"27", [{:feature, :maybe_expr, :enable}]) ++
              if_version_below(~c"28", [{:d, :OTP_BELOW_28}])
     defines = for {:d, value} <- result, do: {:d, value}
     result ++ [{:d, :ALL_DEFS, defines}]

rebar.config

@@ -140,6 +140,7 @@
             {if_version_below, "25", {d, 'OTP_BELOW_25'}},
             {if_version_below, "26", {d, 'OTP_BELOW_26'}},
             {if_version_below, "27", {d, 'OTP_BELOW_27'}},
+            {if_version_below, "27", {feature, maybe_expr, enable}},
             {if_version_below, "28", {d, 'OTP_BELOW_28'}},
             {if_var_false, debug, no_debug_info},
             {if_var_true, debug, debug_info},

src/ext_mod.erl

@@ -678,7 +678,8 @@ maybe_define_lager_macro() ->
     end.
 
 compile_options() ->
-    [verbose, report_errors, report_warnings, debug_info, ?ALL_DEFS]
+    [verbose, report_errors, report_warnings, debug_info, ?ALL_DEFS,
+     {feature, maybe_expr, enable}]
     ++ maybe_define_lager_macro()
     ++ [{i, filename:join(app_dir(App), "include")}
         || App <- [fast_xml, xmpp, p1_utils, ejabberd]]