Update dependency on rules_m4 to fix bazel 9 errors

[jaqx0r]

When using rules_flex with bazel 9, the following error occurs:

ERROR: Traceback (most recent call last):
	File "/home/runner/.cache/bazel/_bazel_runner/dbdc5c7cb47c3a0897d4a4f764f5b22e/external/rules_m4++default_toolchain_ext+m4_v1.4.18/bin/BUILD.bazel", line 10, column 10, in <toplevel>
		cc_binary(
	File "/virtual_builtins_bzl/bazel/exports.bzl", line 40, column 9, in _removed_rule_failure
Error in fail: 
         This rule has been removed from Bazel. Please add a `load()` statement for it.
         This can also be done automatically by running:
         buildifier --lint=fix <path-to-BUILD-or-bzl-file>
         
ERROR: /home/runner/.cache/bazel/_bazel_runner/dbdc5c7cb47c3a0897d4a4f764f5b22e/external/rules_m4++default_toolchain_ext+m4_v1.4.18/bin/BUILD.bazel: no such target '@@rules_m4++default_toolchain_ext+m4_v1.4.18//bin:m4': target 'm4' not declared in package 'bin' defined by /home/runner/.cache/bazel/_bazel_runner/dbdc5c7cb47c3a0897d4a4f764f5b22e/external/rules_m4++default_toolchain_ext+m4_v1.4.18/bin/BUILD.bazel
ERROR: /home/runner/.cache/bazel/_bazel_runner/dbdc5c7cb47c3a0897d4a4f764f5b22e/external/rules_m4++default_toolchain_ext+m4_v1.4.18/rules_m4_internal/BUILD.bazel:4:18: no such target '@@rules_m4++default_toolchain_ext+m4_v1.4.18//bin:m4': target 'm4' not declared in package 'bin' defined by /home/runner/.cache/bazel/_bazel_runner/dbdc5c7cb47c3a0897d4a4f764f5b22e/external/rules_m4++default_toolchain_ext+m4_v1.4.18/bin/BUILD.bazel and referenced by '@@rules_m4++default_toolchain_ext+m4_v1.4.18//rules_m4_internal:toolchain_info'

(from https://github.com/jaqx0r/filtergen/actions/runs/21346251967/job/61434427712)

The latest version of rules_m4 does not have this problem, but rules_flex depends on m4 0.2.5, while the latest is v0.3