Build system improvements

[deminearchiver]

This is a draft pull request, I'll mark it as open as soon as it can be considered ready.

Suggestions and comments are always welcome, though!

This pull request will be the first in a hopefully large set of pull request focusing on improving certain aspects of Obtainium's functionality and development.

For example, I am not ready to contribute to Obtainium if it is not using at least the latest stable Flutter version, as it makes development uncomfortable for me by making me have to install another Flutter version. I think other developers looking to contribute to this project are going to support the initiative of making the developer experience a little bit better.

The main goal of this pull request is to improve the developer experience for contributing to Obtainium without introducing code style and linting changes. I will try to limit the changes to configuration files only. I will not be modifying anything Docker or Bash related.

Changes planned in this pull request:

• [x] Migrate to Flutter 3.38:

  • [x] Change Flutter SDK version constraint to >=3.38.0
  • [x] Bump Java version to 17
  • [x] Bump com.android.application plugin version to 8.11.1
  • [x] Bump org.jetbrains.kotlin.android plugin version to 2.2.20
  • [x] Bump ndkVersion to 28.2.13676358
  • [x] Bump Gradle Wrapper to 8.14

• [x] Migrate to Dart 3.10:

  • [x] Change Dart SDK version constraint to ^3.10.1

• [x] Upgrade background_fetch to 1.5.0:

  • [x] Change background_fetch version constraint to ^1.5.0
  • [x] Remove custom repository in android/build.gradle.kts

• [x] Improve Android build system configuration

  • [x] Make having a keystore.properties file optional for release builds - show a warning during build and use the debug signing config

• [x] Migrate from discontinued flutter_markdown to flutter_markdown_plus