Use an issue form instead of an issue template, fix branch in pull request template

[NetSysFire]

First time contributor checklist

• I have read how to contribute to this project
• I have signed the Contributor License Agreement

Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices: web
• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

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Description

I already did this in the Signal Desktop repo, which you can see in this merged PR: signalapp/Signal-Desktop#6608 - as such I am re-using parts of the issue form and PR description. I got reminded that I only did this for the Signal Desktop repo when I opened an unrelated issue here yesterday. I probably should also look at the other repos and do the same when I have time and motivation.

Trying to create an issue is tedious, especially when you have many multiline comments in the template. Luckily Github has introduced issue forms to make it much easier.
This PR replaces the issue template with an issue form. If you merge this, you do not need to make any additional changes. Github automatically uses the new issue template. Existing issues are completely unaffected and will not be touched.

Before:

After:

You can see for yourself here: https://github.com/NetSysFire/Signal-Android/blob/issue-template/.github/ISSUE_TEMPLATE/bug_report.yaml - alternatively, try to create an issue in the Signal Desktop repo to see how it looks in general.

This is how I also tested that it is working correctly and does not have any syntax errors.
Do note that the descriptions are only visible when writing the issue.

Also you changed this repository default branch to main, but forgot to change the links in the pull request template. I fixed that, too while I was at it.

Additionally, I fixed a tiny whitespace inconsistency that has been annoying me in config.yml.