sync-version: fix warning on gnu grep

[mirthebeijers]

Summary

Without this commit, when using gnu grep (as found on Linuxes), sync-version emits warnings:

$ scripts/sync-version grep: warning: ? at start of expression grep: warning: ? at start of expression grep: warning: ? at start of expression grep: warning: ? at start of expression grep: warning: ? at start of expression grep: warning: ? at start of expression CHANGELOG_VERSION: 0.28.1 VERSION: 0.28.1

I've tested these changes both on Linux and on macOS to make sure they work