Have gnus-summary-limit-to-age operate on calendar days

Reported and patched by Łukasz Stelmach <stlman@poczta.fm>.

* lisp/gnus/gnus-sum.el (gnus-summary-limit-to-age): Rather than
24-hour blocks. Also use `read-number', and drop the confusing option
to reverse younger/older both with the prefix argument and with a
negative number.
1 job for master in 6 minutes and 24 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Normal
failed #75458
test-lisp-gnus-inotify

00:06:24

 
Name Stage Failure
failed
test-lisp-gnus-inotify Normal
Uploading artifacts for failed job
Uploading artifacts...
test-lisp-gnus-inotify-0dd7e6e3/: found 13 matching files and directories

Uploading artifacts as "archive" to coordinator... 201 Created
id=75458 responseStatus=201 Created token=PohRvjCx
Uploading artifacts...
test-lisp-gnus-inotify-0dd7e6e3/junit-test-report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... 201 Created
id=75458 responseStatus=201 Created token=PohRvjCx
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 2