Restore backwards-compatibility infrastructure for 2.0 release branch

[lbianchi-lbl]

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Summary/Motivation:

• Create a version of the IDAES codebase where:
  • (A) All v2 backwards-compatibility infrastructure (BCI) is fully functional (i.e. legacy code will work)
  • (B) All legacy code within IDAES is removed
• To achieve this, the current plan is to:
  • Start from a point in main when most of (B) has been accomplished
  • Revert changes in which BCI was removed to fulfill (A)

Changes proposed in this PR:

• [ ] Add CI checks to ensure (B) is fulfilled
• [ ] Add CI checks to ensure (A) is fulfilled
• [ ] Gradually add back BCI from recent commits (i.e. between 2.0.0a3 and now)

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