Update grsecurity-patched kernels for retbleed patches

[legoktm]

Description

https://comsec.ethz.ch/research/microarch/retbleed/ - once patches are released for stable kernels, we will need to build and release them (currently still needs a SecureDrop release).

A grsecurity patch is now available against thr 5.15.57 kernel, steps to release are as follows:

• Build updated grsec kernel and file a PR to add it to the apt-test repo (Added core 5.15.57 grsec kernel packages securedrop-apt-test#172)
• Bump the kernel version in the securedrop repo (Bump expected kernel version to 5.15.57 #6506)
• Create a 2.4.2 hotfix release containing only the kernel bump
• Cut a 2.4.2-rc1 tag and a packages pr to add them to the apt-test repo
• test the hotfix and kernel update
• Cut a 2.4.2 release and push it to the production repo
• cleanup

Workstation ticket: freedomofpress/securedrop-workstation#809

[legoktm]

5.15.57 dropped today with the retbleed patches: https://lwn.net/Articles/902317/

[zenmonkeykstop]

Now also available in grsec patches as of 28/07.

[legoktm]

2.4.2 was released with an updated kernel.