[18.0][FIX] barcodes_generator_abstract: preserve literal N and D in barcode patterns

[sergiobstoj]

fw-port from #743

[OCA-git-bot]

Hi @legalsylvain,
some modules you are maintaining are being modified, check this out!

[legalsylvain]

/ocabot merge patch

[OCA-git-bot]

What a great day to merge this nice PR. Let's do it!
Prepared branch 18.0-ocabot-merge-pr-748-by-legalsylvain-bump-patch, awaiting test results.

[OCA-git-bot]

@legalsylvain your merge command was aborted due to failed check(s), which you can inspect on this commit of 18.0-ocabot-merge-pr-748-by-legalsylvain-bump-patch.

After fixing the problem, you can re-issue a merge command. Please refrain from merging manually as it will most probably make the target branch red.

[github-actions]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days.
If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.

[legalsylvain]

@sergiobstoj could you take a look on red CI ?