Remove old binutils version pins for gcc 5-9 (#36)

gcc 5-7 were pinned to binutils 2.28 (from a build failure with 2.29
back in Jan 2018). gcc 8/9.1/9.2 were pinned to 2.29.1 when the default
was bumped to 2.34 in March 2020.

The current default is binutils 2.44. Ubuntu 24.04's system libraries
use SHT_RELR (ELF compact relative relocations, type 0x13) which old
binutils doesn't understand, causing link failures when building/running
user code on 24.04 nodes:

  bin/ld: /lib/x86_64-linux-gnu/libm.so.6: unknown type [0x13] section `.relr.dyn'
  bin/ld: cannot find /lib/x86_64-linux-gnu/libm.so.6

Removing these pins so all gcc versions build with the current default
binutils (2.44), which handles SHT_RELR correctly.

🤖 Generated by LLM (Claude, via OpenClaw)

Co-authored-by: mattgodbolt-molty <mattgodbolt-molty@users.noreply.github.com>

Author: mattgodbolt-molty