Downgrade Vue.js to 2.6.14 and vue-loader to 15.9.6

What does this MR do and why?

Our recent upgrade dramatically increased the compilation of our assets and the reason for that these two package: vue and vue-loader. To continue investigating the root case we need to downgrade these packages to previous versions to fix the compile-production-assets job.

Closes #371244 (closed)

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

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.