Golang cmd/covdata: too many open files due to defer f.Close() in for loop [1.21 backport]

@thanm requested issue #68468 to be considered for backport to the next 1.21 minor release.

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

Comment From: gopherbot

Change https://go.dev/cl/599055 mentions this issue: cmd/internal/cov: close counter data files eagerly

Comment From: gopherbot

Closed by merging CL 599055 (commit dabed2e09a888ec33b385006890d3c66f2ced8ca) to release-branch.go1.21.