Pin definitions when building an older version

[kfranqueiro]

This is a follow-up to #4007, in which I had neglected to pin definitions for key terms to the published recommendation when building for 2.1.

• Moves termsMap definition to the Eleventy config (where all other target-version-dependent assignments are made), to be passed into the CustomLiquid constructor
• Updates termsMap references within CustomLiquid to use an instance variable set in the constructor
• Updates loadRemoteGuidelines to account for pre-processed terms content
  • Also updates to account for Add build support for linking to bibliographical references #4124
• Skips entirety of custom render processing for pages not being output for 2.1 (to avoid irrelevant error messages as well as save time)

[w3cbot]

kfranqueiro marked as non substantive for IPR from ash-nazg.

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