(docs) Update upgrade guide with clarifications on vueOptionsAPI

[DaleMckeown]

The change to default vueOptionsAPI to false caught me out, despite it not being used directly in my project. Another dependency was still using the options API, and I had to re-enable this to fix. I wanted to update the docs to make others think about their other dependencies and whether they need to re-enable this option.

Clarified the default value and implications of vueOptionsAPI in the upgrade guide. Updated notable breaking changes to include additional context for vueOptionsAPI and dotenv support.

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to start a new feature discussion first and wait for approval before working on it)

Other information: