Golang runtime: heap mspan limit is set too late, causing data race between span allocation and conservative scanning [1.23 backport]

@mknyszek requested issue #74288 to be considered for backport to the next 1.23 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.