[v12] fix: use errors.Is for all EOF comparisons

[nklaassen]

Backport #26012 to branch/v12

This commit updates all err == io.EOF comparisons to use errors.Is(err, io.EOF). This is necessary when the error may have been wrapped and fixes at least one current breakage (in tsh request ls).

golang.org/x/tools/refactor/eg was very handy for this, I used the following template:

package teleport
import (
	"errors"
	"io"
)
func before(err error) bool { return err == io.EOF }
func after(err error) bool  { return errors.Is(err, io.EOF) }