runtime: modified timer results in extreme cpu load [1.18 backport]

[gopherbot]

@prattmic requested issue #51654 to be considered for backport to the next 1.18 minor release.

@gopherbot Please backport to 1.17 and 1.18. This is a serious issue (busy loop in scheduler for up to 2 minutes) with no reasonable workaround.

[gopherbot]

Change https://go.dev/cl/417475 mentions this issue: [release-branch.go1.18] runtime: clear timerModifiedEarliest when last timer is deleted

[toothrot]

Approved. This is a serious issue with no workaround.

[gopherbot]

Closed by merging 4782f42 to release-branch.go1.18.