Stop versioning composer.lock

Problem/Motivation

composer.lock was versioned because it contained some dev dependencies used in pre-commit checs and we wanted to ensure every contributor used the same version of the checks.
For ✨ Webform XLSX Export Module to Support PHPSpreadsheet ^2.1 Fixed we had to remove these dev dependencies so versioning the composer.lock file is not useful anymore and generates harder to read patches for changes like 🐛 XXE in PHPSpreadsheet's XLSX reader Active .

Proposed resolution

Remove the composer.lock file from the repository and add it to .gitignore.