Support period lengthened for the 6.6, 6.12, and 6.18 kernels

The stated support periods for the 6.6, 6.12, and 6.18 kernels has been extended.
The 6.6 kernel will be supported with stable updates through the end of
2027 (for four years of support total), while 6.12 and 6.18 will get
updates through the end of 2028, for four and three years of support.