Accept text like '_posts/2024-03-05-wildfly-gitops.md' as the value for the update_post workflow post_name param

[bstansberry]

Very minor suggestion for the update_post workflow:

Allow for an (optional) preceding '_posts/' in the value for the update_post workflow post_name param.

Why?

If you go to the 'files' view for PR that might need an update, e.g.

https://github.com/wildfly/wildfly.org/pull/562/files

In the heading for single file in the PR the Github UI has a icon that lets you copy the file name, e.g.

_posts/2024-03-05-wildfly-gitops.md

Click that and the pasting the value into the update_post workflow dialog would be nice.

Very minor as it's hardly a big imposition to paste and then edit to prune the preceding '_posts/'