Golang cmd/covdata: too many open files due to defer f.Close() in for loop [1.22 backport]

@thanm requested issue #68468 to be considered for backport to the next 1.22 minor release.

@gopherbot please consider this for backport to 1.22 and 1.21.

Comment From: gopherbot

Change https://go.dev/cl/599036 mentions this issue: cmd/internal/cov: close counter data files eagerly

Comment From: gopherbot

Closed by merging CL 599036 (commit 4c50f9162cafaccc1ab1bc26b0dea18f124b536d) to release-branch.go1.22.