Add a check that confirms that license section is not removed from the PR description

[marko-bekhta]

With the new pull request templates containing a section about the user confirming the license under which the contribution is made, e.g.

https://github.com/hibernate/hibernate-search/blob/1500a1f243786a7d17294c41879ec00f13a4f009/.github/PULL_REQUEST_TEMPLATE.md?plain=1#L17-L22

We should let the bot check that the section is not removed, and if it is removed -- fail the check and add a message explaining that the license section should be present.

We should consider making this check optional for Hibernate Team members (if possible).