net-test: adjust 'accept_ra_min_lft' minimum LTS kernel version check

The following 3 Linux 6.6 patches:
  5cb249686e67dbef3ffe53887fa725eefc5a7144 net: release reference to inet6_dev pointer
  5027d54a9c30bc7ec808360378e2b4753f053f25 net: change accept_ra_min_rtr_lft to affect all RA lifetimes
  1671bcfd76fdc0b9e65153cf759153083755fe4c net: add sysctl accept_ra_min_rtr_lft

are present in 6.1.57:
  b6912642746d2c48b80c205b26145839a3a412ed net: release reference to inet6_dev pointer
  bad004c384b7f15fa84a6613868c85e3efcf016e net: change accept_ra_min_rtr_lft to affect all RA lifetimes
  ec4162bb70189648ac39dd5a13d4f16e78b15544 net: add sysctl accept_ra_min_rtr_lft

and 5.15.136:
  5e13e69ddf0d70fd4caf1ed1608e03520a39b4f6 net: release reference to inet6_dev pointer
  aade10d51ddc109a9ae2a88c5b6f80d956b5dc01 net: change accept_ra_min_rtr_lft to affect all RA lifetimes
  8f12d2d66cba6535362e2f8dc57808dfb45c7cc4 net: add sysctl accept_ra_min_rtr_lft

and 5.10.199:
  014cab5329d18524749dacc25923938aab14ff57 net: release reference to inet6_dev pointer
  d491ac7aa12a09a1a48f9f6cbf628b29080ca9f4 net: change accept_ra_min_rtr_lft to affect all RA lifetimes
  354a96770d94ce2cf2c9d4671b19401641877e9f net: add sysctl accept_ra_min_rtr_lft

Test: via UML on ACK 6.1 V (@6.1.71)
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ibf5f216fbccc431a8f88f3360b9f6873098b0796