Improve auto-update

[MrAdityaAlok]

TODO:

• [ ] Make termux_{github,gitlab}_get_tag use inherited variables.
• [ ] Fix latest-regex behaviour. Also add for gitlab.
• [ ] Refactor TERMUX_GITLAB_API_HOST use.
• [ ] And few syntactical changes.

[TomJo2000]

• Hi, I might be causing some merge conflicts for this as part of refactoring the gitlab update method to fix #27566.

Just wanted to give you a heads up, I'll try not to cause you too many headaches. This largely intersects with what you're doing here.