chore: update plug_cowboy

recently `mix deps.audit` started to fail due to `plug_cowboy` being affected by
a severe vulnerability.

Signed-off-by: Luca Zaninotto <luca.zaninotto@secomind.com>

Author: lusergit

backend/mix.lock

@@ -85,7 +85,7 @@
   "phoenix_view": {:hex, :phoenix_view, "2.0.4", "b45c9d9cf15b3a1af5fb555c674b525391b6a1fe975f040fb4d913397b31abf4", [:mix], [{:phoenix_html, "~> 2.14.2 or ~> 3.0 or ~> 4.0", [hex: :phoenix_html, repo: "hexpm", optional: true]}, {:phoenix_template, "~> 1.0", [hex: :phoenix_template, repo: "hexpm", optional: false]}], "hexpm", "4e992022ce14f31fe57335db27a28154afcc94e9983266835bb3040243eb620b"},
   "picosat_elixir": {:hex, :picosat_elixir, "0.2.3", "bf326d0f179fbb3b706bb2c15fbc367dacfa2517157d090fdfc32edae004c597", [:make, :mix], [{:elixir_make, "~> 0.6", [hex: :elixir_make, repo: "hexpm", optional: false]}], "hexpm", "f76c9db2dec9d2561ffaa9be35f65403d53e984e8cd99c832383b7ab78c16c66"},
   "plug": {:hex, :plug, "1.19.1", "09bac17ae7a001a68ae393658aa23c7e38782be5c5c00c80be82901262c394c0", [:mix], [{:mime, "~> 1.0 or ~> 2.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.1.1 or ~> 1.2 or ~> 2.0", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4.3 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "560a0017a8f6d5d30146916862aaf9300b7280063651dd7e532b8be168511e62"},
-  "plug_cowboy": {:hex, :plug_cowboy, "2.8.0", "07789e9c03539ee51bb14a07839cc95aa96999fd8846ebfd28c97f0b50c7b612", [:mix], [{:cowboy, "~> 2.7", [hex: :cowboy, repo: "hexpm", optional: false]}, {:cowboy_telemetry, "~> 0.3", [hex: :cowboy_telemetry, repo: "hexpm", optional: false]}, {:plug, "~> 1.14", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm", "9cbfaaf17463334ca31aed38ea7e08a68ee37cabc077b1e9be6d2fb68e0171d0"},
+  "plug_cowboy": {:hex, :plug_cowboy, "2.8.1", "5aa391a5e8d1ac3192e36a3bcaff12b5fd6ef6c7e29b53a38e63a860783e77d0", [:mix], [{:cowboy, "~> 2.7", [hex: :cowboy, repo: "hexpm", optional: false]}, {:cowboy_telemetry, "~> 0.3", [hex: :cowboy_telemetry, repo: "hexpm", optional: false]}, {:plug, "~> 1.14", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm", "4c200288673d5bc86a0ab7dc6a2a069176a74e5d573ef62740a1c517458a5f26"},
   "plug_crypto": {:hex, :plug_crypto, "2.1.1", "19bda8184399cb24afa10be734f84a16ea0a2bc65054e23a62bb10f06bc89491", [:mix], [], "hexpm", "6470bce6ffe41c8bd497612ffde1a7e4af67f36a15eea5f921af71cf3e11247c"},
   "plug_heartbeat": {:hex, :plug_heartbeat, "1.0.0", "11b263a4c04d45b85c16ae815dd9a29ad405d7962769313c1244959328ebc39c", [:mix], [{:plug, "~> 1.0", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm", "fa17f6eac7f4d91fcef36d253bc0114ff8c4d6e91665f2b06be1beb753cbb6aa"},
   "poison": {:hex, :poison, "4.0.1", "bcb755a16fac91cad79bfe9fc3585bb07b9331e50cfe3420a24bcc2d735709ae", [:mix], [], "hexpm", "ba8836feea4b394bb718a161fc59a288fe0109b5006d6bdf97b6badfcf6f0f25"},