bump the longterm EOL dates to a be a bit longer than previously documented

Based on lots of discussions with different companies and groups and the
other stable kernel maintainer, this moves to show that the current
status is:
  5.10 to be supported for 6 years
  5.15 to be supported for 5 years
  6.6 to be supported for 4 years
  6.12 to be supported for 4 years
  6.18 to be supported for at least 3 years

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>