Remove references to scipopt.org; update DoE paper reference

[jsiirola]

Fixes # .

Summary/Motivation:

The scipopt.org web server now aggressively disallows automated connections. As a result, the URL verification action is consistently failing. This PR removes all references to scipopt.org removes any "deep" references to scipopt.org (e.g., to documentation or license) from the repository and documentation.

CC (for awareness): @scipopt

While we were updating URLs, this tracks a change in the DOI link for the DoE paper, and moves the internal reference in the Pyomo docs from the bibliography to the publications list.

Changes proposed in this PR:

• Remove all references to scipopt.org except for https://www.scipopt.org
• Exclude https://www.scipopt.org from URL validation
• Update DOI for the Pyomo.DOE paper
• Move internal citation for the Pyomo.DoE paper from "bibliography" to "publications"

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