Improve weekly baseline version handling

[sghill]

Noticed while opening PRs plugins will sometimes depend on a weekly version and be satisfied with falling back to the most recent LTS bom. In this case it makes sense to leave the recent bom alone.

jenkinsci/cloudbees-folder-plugin#415

What did you expect to see?

<dependency>
    <groupId>io.jenkins.tools.bom</groupId>
    <artifactId>bom-2.440.x</artifactId>
    <version>2982.vdce2153031a_0</version>
    <scope>import</scope>
    <type>pom</type>
</dependency>

What did you see instead?

<dependency>
    <groupId>io.jenkins.tools.bom</groupId>
    <artifactId>bom-weekly</artifactId>
    <version>3234.v5ca_5154341ef</version>
    <scope>import</scope>
    <type>pom</type>
</dependency>

[jglick]

Actually in this case I would expect it to go to the 2.462.x BOM, and adjust jenkins.version accordingly.

See also jenkinsci/archetypes#737 and jenkinsci/workflow-multibranch-plugin#314.