Remove reference to Prototype

[basil]

When running on recent releases of Jenkins core with Prototype removed, a scary-looking but harmless warning (stack trace) is printed to the Jenkins log:

2023-04-04 03:12:57.973+0000 [id=264]        WARNING        o.k.s.f.adjunct.AdjunctsInPage#findNeeded: No such adjunct found: org.kohsuke.stapler.framework.prototype.prototype
org.kohsuke.stapler.framework.adjunct.NoSuchAdjunctException: Neither org.kohsuke.stapler.framework.prototype.prototype.css, .js, .html, nor .jelly were found
        at org.kohsuke.stapler.framework.adjunct.Adjunct.<init>(Adjunct.java:125)
        at org.kohsuke.stapler.framework.adjunct.AdjunctManager.get(AdjunctManager.java:148)
        at org.kohsuke.stapler.framework.adjunct.AdjunctsInPage.findNeeded(AdjunctsInPage.java:161)
        at org.kohsuke.stapler.framework.adjunct.AdjunctsInPage.assumeIncluded(AdjunctsInPage.java:131)
        at org.kohsuke.stapler.framework.adjunct.AdjunctsInPage.assumeIncluded(AdjunctsInPage.java:125)

This PR updates the core baseline to the (forthcoming) LTS release (2.426.x), which removes the harmless warning.

Testing done

Reproduced the warning on a recent weekly, and verified that with this change the warning is no longer printed.

[mtughan]

Thanks @basil. Will try to get to this in the next few days and get it merged shortly.