Update LTS version in channels.json to reflect 7.4

[anamnavi]

PR Summary

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all Dockerfile, .sh, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[jakauppila]

Curious when this will be completed and if there could be communication going forward on when this will occur as new LTS releases come out?