Use ISO8601 time formats in tar-mode

* lisp/tar-mode.el (tar-clip-time-string): Make obsolete (bug#37130).
(tar-header-block-summarize): Use ISO8601 time instead of
home-brew format.
1 job for master in 90 minutes and 2 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #3087
test-all

01:30:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds