Update main to v8.0.0.alpha, dropping deprecated behaviors

[cbeer]

This is mostly a rebase of #2393 with updates to get it current, but dropped a couple commits that we might want to visit some day:

• 5065c03 (super conflict-y and needs to be redone)
• dbcdfd2 (possibly not needed)

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Beyond dropping deprecated methods and behavior from v7, this PR also:

• bumps the minimum view_component version and removes some of the blacklight-custom shims to maintain backwards compatibility with older view component APIs
• drops support for Rails 5.2 (which.. may not be a strict requirement, but Rails 5.2 and 7.x will probably be incompatible)
• drops some of the default actions configuration that must be explicitly configured in the local application (generated by default for quite some time)

[tpendragon]

@cbeer CI is failing, how can I most effectively help make this happen? Should we just put on the committer call an agenda item for making Blacklight 8 the main branch?

[cbeer]

I've rebased this and it's close to passing. Those commits that needed review or reworking still need some attention, and needs some work to get the pile of ugly commits at the end re-integrated into the earlier commits where appropriate.

This PR also requires #2516 to get merged first first, in order to fix up some behavior that is deprecated and is removed in this PR.