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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.11%. Comparing base (cf43df0) to head (4a52048).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3968      +/-   ##
==========================================
- Coverage   90.11%   90.11%   -0.01%     
==========================================
  Files         909      909              
  Lines      108561   108561              
==========================================
- Hits        97835    97832       -3     
- Misses      10726    10729       +3     

Flag	Coverage Δ
builders	29.11% <ø> (+<0.01%)	⬆️
default	86.11% <ø> (?)
expensive	35.12% <ø> (?)
linux	87.63% <ø> (-1.99%)	⬇️
linux_other	87.63% <ø> (ø)
oldsolvers	28.04% <ø> (+<0.01%)	⬆️
osx	83.05% <ø> (ø)
win	85.37% <ø> (-0.01%)	⬇️

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