Fix concurrent modification exception in ExecuteDslScripts

[gtaylor1981]

This PR fixes an intermittant problem we've been seeing on our builds, where alternate builds fail because of ConcurrentModificationException in ExecuteDslScripts. This was raised as JENKINS-69064.

Explanation
As part of the cleanup of removed templates in the updateTemplates() method, all seed job references are read for the removed template. These references are then filtered to those matching the seed job name. These matches are then removed from the seed job references.

However, the filtered references are a 'live' view (from Collections2.filter) based on the original seed job references, so attempting to remove them results in a ConcurrentModificationException.

Fix
The fix simply first converts the 'live' matches collection to a hash set before passing it to removeAll(), which allows the original collection to be modified. A unit test has been added to confirm this.

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[jamietanna]

Thanks for tracking this down and resolving it 🚀