Fixed: Revert `desugar_jdk_libs` back to 1.1.5

- This is necessary because `desugar_jdk_libs` version 2.1.2 adds new `java.nio.file` APIs that are untested, and additionally, `java.nio.file` APIs are known to be broken, so will be removed from Termux in the future rather than being bumped. More information here: https://github.com/termux/termux-app/pull/3619#discussion_r2680770189

- This is necessary to prevent errors like this while using the [local Maven repository guide](https://github.com/termux/termux-app/wiki/Termux-Libraries#forking-and-local-development) to build reverse dependencies like Termux:API after https://github.com/termux/termux-app/pull/3619

```
1.  Dependency 'com.termux:termux-shared:0.118.0' requires desugar_jdk_libs version to be
           2.1.2 or above for :app, which is currently 1.1.5
```

- Partially reverts https://github.com/termux/termux-app/pull/3619