Linux: bump minimum kernel version to 4.18

[robn]

Motivation and Context

I started cutting things out in #16434. Everyone loved it, and wanted more. I'm nothing if not a crowd pleaser!

Description

Just like last time, but this time the minimum is 4.18, as the base kernel for RHEL8.

Since last time, more stuff removed. Following comments there, I've left DECLARE_EVENT_CLASS, and the baseline checks.

How Has This Been Tested?

Compile checked on kernel.org kernels:

• 4.19.319
• 5.4.281
• 5.10.223
• 6.1.102
• 6.2.16
• 6.3.13
• 6.5.13
• 6.6.43
• 6.7.12
• 6.8.12
• 6.9.12
• 6.10.2
• 6.11-rc2

And on following distros:

• 5.4.0-84-generic (Ubuntu 18.04)
• 5.13.0-30-generic (Ubuntu 20.04)
• 5.15.0-25-generic (Ubuntu 22.04)
• 6.8.0-31-generic (Ubuntu 24.04)
• 4.18.0-348.7.1.el8_5.x86_64 (CentOS 8.4.2105)
• 4.18.0-553.8.1.el8_10.x86_64 (Rocky 8.6)
• 5.14.0-427.28.1.el9_4.x86_64 (Rocky 9)
• 6.10.5-200.fc40.x86_64 (Fedora 40)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Performance enhancement (non-breaking change which improves efficiency)
• [x] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• [ ] Documentation (a change to man pages or other documentation)

Checklist:

• [x] My code follows the OpenZFS code style requirements.
• [ ] I have updated the documentation accordingly.
• [x] I have read the contributing document.
• [ ] I have added tests to cover my changes.
• [ ] I have run the ZFS Test Suite with this change applied.
• [x] All commit messages are properly formatted and contain Signed-off-by.