Replace Set.card() with Set.at()

[jsiirola]

Fixes # .

Summary/Motivation:

@eslickj correctly pointed out that card() was NOT the correct replacement for Set.__getitem__ (retrieving an item from an ordered Set by position). This PR corrects the mistake from #2053 and switches card() to at().

Changes proposed in this PR:

• Replace Set.card() with Set.at()

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

Merging #2096 (77717b8) into main (828c551) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2096   +/-   ##
=======================================
  Coverage   82.87%   82.88%           
=======================================
  Files         619      619           
  Lines       76848    76851    +3     
=======================================
+ Hits        63689    63696    +7     
+ Misses      13159    13155    -4     

Impacted Files	Coverage Δ
pyomo/core/base/set.py	99.67% <100.00%> (+<0.01%)	⬆️
pyomo/dae/contset.py	98.96% <100.00%> (ø)
pyomo/contrib/mindtpy/iterate.py	76.73% <0.00%> (+1.98%)	⬆️

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