Remove bwc state loading logic in MetaStateService

[martijnvg]

As part of preparing master for 8.0 (#38514), the assertions were disabled in MetaStateService#loadFullStateBWC(...) were disabled.

In an effort to make 8.0 build pass quickly, however looking at this issue, besides removing the mentioned method and loading just an empty manifest if there is none, also some tests need to change. For example RecoveryWithUnsupportedIndicesIT#testUpgradeStartClusterOn_2_4_5, I think it should now try to load a 5.x index?

This bwc logic was added via #35049.

[elasticmachine]

Pinging @elastic/es-distributed

[martijnvg]

Closing in favor of #38556