fix(min-publish-age): remove registry.min-publish-age

[weihanglo]

What does this PR try to resolve?

This caused some confusions.
Better to stabilize min-publish-age configs incrementally.

The main one-way-door we will take here is the agreement on the global- prefix for global settings in the [registry] table. For more discussions, see
https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/.5Bregistry.5D.20vs.20.5Bregistries.2Ecrates-io.5D/with/616084717

How to test and review this PR?

CI passes.

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The reviewer was selected based on:

• Owners of files modified in this PR: @epage, @weihanglo
• @epage, @weihanglo expanded to epage, weihanglo

[weihanglo]

---- resolver::version_prefs::test::publish_age_zero_stops_scope_fallthrough stdout ----

thread 'resolver::version_prefs::test::publish_age_zero_stops_scope_fallthrough' (37825) panicked at src/resolver/version_prefs.rs:749:9:
assertion `left == right` failed
  left: Some(PublishAgeViolation { age: 0s, config: "30 days" })
 right: None

---- resolver::version_prefs::test::publish_age_crates_io_scope_excludes_alt_registry stdout ----

thread 'resolver::version_prefs::test::publish_age_crates_io_scope_excludes_alt_registry' (37813) panicked at src/resolver/version_prefs.rs:688:9:
assertion `left == right` failed
  left: None
 right: Some(PublishAgeViolation { age: 48h, config: "30 days" })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    resolver::version_prefs::test::publish_age_crates_io_scope_excludes_alt_registry
    resolver::version_prefs::test::publish_age_zero_stops_scope_fallthrough

Hmm, it seems to discover a real bug.

[weihanglo]

---- resolver::version_prefs::test::publish_age_zero_stops_scope_fallthrough stdout ----

thread 'resolver::version_prefs::test::publish_age_zero_stops_scope_fallthrough' (37825) panicked at src/resolver/version_prefs.rs:749:9:
assertion ``left == right`` failed
  left: Some(PublishAgeViolation { age: 0s, config: "30 days" })
 right: None

---- resolver::version_prefs::test::publish_age_crates_io_scope_excludes_alt_registry stdout ----

thread 'resolver::version_prefs::test::publish_age_crates_io_scope_excludes_alt_registry' (37813) panicked at src/resolver/version_prefs.rs:688:9:
assertion ``left == right`` failed
  left: None
 right: Some(PublishAgeViolation { age: 48h, config: "30 days" })
note: run with ``RUST_BACKTRACE=1`` environment variable to display a backtrace


failures:
    resolver::version_prefs::test::publish_age_crates_io_scope_excludes_alt_registry
    resolver::version_prefs::test::publish_age_zero_stops_scope_fallthrough

Hmm, it seems to discover a real bug.

Fixed by a fallback. Non-deterministic hashmap + our SourceId hash key impl strike again.