fix: require kernel 5.8, the first with BPF ring buffers

[mayankpande88]

Summary

L7 events go through a BPF ring buffer (BPF_MAP_TYPE_RINGBUF), which every program variant uses. Kernels before 5.8 don't have one. The startup check still accepted kernels from 4.16, so on 4.16–5.7 the agent got past it, initialized, and then failed loading the programs with map l7_events: map create: invalid argument. The README gave 5.1 as the minimum.

The check and the README now say 5.8, and the error names the reason.

Engineering detail

• The 4.16, 4.20 and 5.6 program variants all contain the ring buffer map, so none of them loads below 5.8.
• The variant named "5.6" is the one kernels 5.8–5.11 use, so it stays. Removing the 4.16 and 4.20 variants from the bundle would shrink the binary; that's left for a separate change.

Local e2e: built the agent from this branch and from main against glibc 2.31 and ran both on a 5.4 kernel VM (Ubuntu 20.04, arm64). main: passed the version check, initialized for 4s, then failed to load collection: map l7_events: map create: invalid argument. This branch: exits at once with the minimum Linux kernel version required is 5.8 or later (BPF ring buffer). On a 6.1 kernel VM this branch starts normally and serves its metrics.

[gemini-code-assist]

Code Review

This pull request updates the minimum required Linux kernel version from 4.16 to 5.8 in main.go and from 5.1 to 5.8 in README.md. This change is necessary because L7 events now utilize a BPF ring buffer (BPF_MAP_TYPE_RINGBUF), which is unsupported on kernels older than 5.8. I have no feedback to provide.