Be more explicit on flutter version change

### Page URL

https://docs.flutter.dev/install/upgrade/

### Page source

https://github.com/flutter/website/tree/main/src/content/install/upgrade.md

### Describe the problem

Instructions for "Switch to a specific Flutter version" are unclear. They might be ok for people who don't know anything about building applications so that they can copy-paste solutions, or they are great for people who know how exactly the flutter works. As someone who occasionally dibbles with flutter, it was confusing to me why ONLY `git checkout` in a local repo would affect the global executable. Likewise, it isn't clear to me that the expected version is just the symver, i.e. `3.29.3`. Often these can be `v3.29.3` or specific hash.

### Expected fix

1. Add a line that "once the repository is updated execution of any flutter command will update the flutter to the expected version".
2. Add an example of the version expected, e.g. "to change to version 3.32.0 type `git checkout 3.32.0`".

Bonus:
* It's now recommended to use `git switch` instead of `git checkout`.

### Additional context

_No response_

### I would like to fix this problem.

- [x] I will try and fix this problem on docs.flutter.dev.