Add version to pdf.js file in webpack builds

What does this MR do and why?

adding version to the webpack builds for pdf.js files, as part of the corrective action after gitlab-com/gl-infra/production#18574 (comment 2113462867). this follows the existing format for @sourcegraph and @gitlab/web-ide

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

• follow the instructions to run the app in webpack
• run yarn run clean to remove the existing pubic/assets files
• run yarn webpack, new pdfjs files should be copied under public/assets/webpack/pdfjs/3.11.174
• viewing pdf files should work as expected

Related to #491716 (closed)