runtime: heap `mspan` limit is set too late, causing data race between span allocation and conservative scanning [1.24 backport]

[gopherbot]

@mknyszek requested issue #74288 to be considered for backport to the next 1.24 minor release.

@gopherbot Please open backport issues for Go 1.23 and Go 1.24.

This problem can cause rare random crashes in the runtime with no workaround.

[gopherbot]

Change https://go.dev/cl/684079 mentions this issue: [release-branch.go1.24] runtime: set mspan limit field early and eagerly

[gopherbot]

Closed by merging CL 684079 (commit 6b51660) to release-branch.go1.24.