build(deps): update xema requirement from 0.13.6 to 0.13.7

Updates the requirements on [xema](https://github.com/hrzndhrn/xema) to permit the latest version.
- [Release notes](https://github.com/hrzndhrn/xema/releases)
- [Changelog](https://github.com/hrzndhrn/xema/blob/master/CHANGELOG.md)
- [Commits](https://github.com/hrzndhrn/xema/commits/v0.13.7)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

mix.exs

@@ -79,7 +79,7 @@ defmodule StepFlow.MixProject do
       {:prometheus_plugs, "~> 1.1"},
       {:slack, "~> 0.23.5"},
       {:timex, "~> 3.2"},
-      {:xema, "0.13.6"}
+      {:xema, "0.13.7"}
     ]
   end