Replace configuration from environment variables by `configs` top-level element

[louisroyer]

• https://docs.docker.com/reference/compose-file/configs/

• https://docs.docker.com/reference/compose-file/services/#configs

• Update Dockerfiles to drop template-scripts

  • nextmn/upf
  • nextmn/srv6
  • nextmn/srv6-ctrl
  • nextmn/ue-lite
  • nextmn/gnb-lite
  • nextmn/cp-lite
  • free5gc-populate
  • free5gc (amf, smf, ausf, chf, nrf, nssf, udm, udr, pcf, webconsole, upf)
  • ueransim (ue, gnb)