Link release note issues to GitLab issue URLs

We indicate issue numbers for issues in the release notes, but these can't be clicked. It would be nicer if they were links.

I see multiple ways to do this:

• Make each issue number a link: https://gitlab.eclipse.org/eclipse/escet/escet/-/issues/396[#396]. Unfortunately, this is quite long.
• Use an attribute. First, define once: :escet-issue: https://gitlab.eclipse.org/eclipse/escet/escet/-/issues/. Then, use it to create an issue link: {escet-issue}396[#396]. It is much shorter, but unfortunately requires listing the issue number twice, as an attribute can't have a parameter. AsciiDoc does not seem to have parametrized attributes, macros, or so.

We may then also remove the following line from the release note pages:

The release notes may refer to issues, the details for which can be found at the Eclipse ESCET link:https://gitlab.eclipse.org/eclipse/escet/escet/-/issues[GitLab issues page].