integrate EC membership changes into upgrade handler

[dckc]

What is the Problem Being Solved?

Following a signalling proposal, replace-electorate-core.js is expected to execute as part of an upcoming chain-halting upgrade. So it should be included in upgrade.go

(originally discussed in #10251 (comment) )

Description of the Design

Somewhat like dd97f18 but in reverse.

Security / Documentation / Upgrade Considerations

It's critical that the updated list of addresses are actually controlled by the Economic Committee membership and noone else.

The Economic Committee is elected by the BLD stakers; a signalling proposal should provide documentation to BLD stakers that the chain-halting upgrade proposal is also a proposal to elect an updated slate of committee members.

Scaling Considerations

Test Plan

a3p testing such as in #10241 should use a SoftwareUpgradeProposal, not a CoreEvalProposal