compat: Simulate Vue 2.6.14 version in vue-compat

[CyberAP]

Some Vue plugins that are actually compatible with the vue-compat won't work because of the version check (an example from vue apollo). Right now vue-compat returns 3x version for the legacy method of checking Vue version via Vue.version. This PR instends to simulate Vue 2 version in those instances and instead return a 2x-compatible version: 2.6.14-compat:3.x.x.

[netlify]

✔️ Deploy Preview for vuejs-coverage ready!

🔨 Explore the source changes: 0c9cc89

🔍 Inspect the deploy log: https://app.netlify.com/sites/vuejs-coverage/deploys/61e901cde7e62700074d0645

😎 Browse the preview: https://deploy-preview-5293--vuejs-coverage.netlify.app

[netlify]

✔️ Deploy Preview for vue-next-template-explorer ready!

🔨 Explore the source changes: 0c9cc89

🔍 Inspect the deploy log: https://app.netlify.com/sites/vue-next-template-explorer/deploys/61e901cd256da20008c16f4f

😎 Browse the preview: https://deploy-preview-5293--vue-next-template-explorer.netlify.app

[netlify]

✔️ Deploy Preview for vue-sfc-playground ready!

🔨 Explore the source changes: 0c9cc89

🔍 Inspect the deploy log: https://app.netlify.com/sites/vue-sfc-playground/deploys/61e901cd79ea080007108ba8

😎 Browse the preview: https://deploy-preview-5293--vue-sfc-playground.netlify.app/