Stop versioning composer.lock

Problem/Motivation

composer.lock was versioned because it contained some dev dependencies used in pre-commit checks and we wanted to ensure every contributor used the same version of the checks.
For #3470891: Webform XLSX Export Module to Support PHPSpreadsheet ^2.1 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 #3479278: XXE in PHPSpreadsheet's XLSX reader.

Proposed resolution

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