fix(transparent-proxy): make iptables mode detection more defensive (backport of #9776)

[kumahq]

Automatic cherry-pick of #9776 for branch release-2.3

Generated by action

cherry-picked commit fdebf4b
⚠️ ⚠️ ⚠️ Conflicts happened when cherry-picking! ⚠️ ⚠️ ⚠️

On branch release-2.3
Your branch is up to date with 'origin/release-2.3'.

You are currently cherry-picking commit fdebf4b7d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   pkg/transparentproxy/iptables/builder/builder_raw.go
	modified:   pkg/transparentproxy/iptables/setup.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   pkg/transparentproxy/config/config.go
	both modified:   pkg/transparentproxy/iptables/builder/builder.go
	deleted by us:   pkg/transparentproxy/iptables/builder/builder_restore.go