mev-boost already has X-MEVBoost-StartTimeUnixMS but ethereum/builder-specs#106 , the issue formalizing this behavior in the spec ended up settling on a slightly different name 😬 . bike shedding some naming humans still do best 😄.

as discussed in private with @metachris , to make the transition smooth it would be nice if mev-boost released one version with both the current header people may already depend on and the spec'd header. then in the next release drop the old naming.

this behavior is landing soon in vouch: attestantio/vouch#292
and has already landed in commit-boost: Commit-Boost/commit-boost-client#259

i'll open one PR to add the new header
and one draft PR to drop the current one that can be merged after the next release.