Inconsistency between CI/CD and README versions

[RichDom2185]

In our CI/CD pipelines, we see the following:

• backend/.github/workflows/ci.yml

  Lines 26 to 27 in 15e12d8

  	ELIXIR_VERSION: "1.13.3"
  	OTP_VERSION: "24.2.1"

• backend/.github/workflows/ci.yml

  Line 30 in 15e12d8

  image: postgres:14.2

But in our README for development:

• backend/README.md

  Lines 17 to 19 in 15e12d8

  	1. Elixir 1.13.3
  	2. Erlang/OTP 23.2.1
  	3. PostgreSQL 13 or 14

Note that the README shows Erlang/OTP 23 while the actual build uses Erlang/OTP 24. Judging by the commit/PR history, it seems like even during the upgrade to OTP 24 (#856), the inconsistency was already there to begin with.

Let us make these versions consistent to potentially catch compatibility bugs during development.