Accept vanilla Linux Foundation signoffs (Closes 7368)

Closes #7368
Oh wait, I just realized this variable is used in the output below.  We probably want to just move `(Docker-DCO-1.1-)?` directly into `dcoRegex` and have this stay as just `Signed-off-by:`, ie:
```bash
dcoPrefix='Signed-off-by:'
dcoRegex="^(Docker-DCO-1.1-)?$dcoPrefix ([^<]+) <([^<>@]+@[^<>]+)>( \\(github: ($githubUsernameRegex)\\))?$"
```