linux-image: Add retrospective NEWS entry for the removal of XFS V4 support

XFS introduced the V5 filesystem format long ago (fully supported since 3.15). It added support for metadata CRCs and an extensible inode structure that allows for timestamps >2038.

The previous V4 format has been deprecated since 5.10, with support conditional on CONFIG_XFS_SUPPORT_V4. Linux 6.18 changed the upstream default for the symbol from y to n, and since we did not set a value for it we followed that change. This change was reported as Debian bugs #1126710 and #1128422.

The kernel team has agreed to follow the upstream change rather than overriding it. V5 has been the default since stretch, while V4 will be completely removed upstream in 2030, making it unsupportable in the next stable release.

Document this change and its consequences, and explain how to check the XFS format version.