Raise minSdkVersion from 21 to 26 in the next major release

## Summary

We plan to raise the minimum supported Android SDK version from **21** (Lollipop 5.0) to **26** (Oreo 8.0) in the next major version of `sentry-android`.

We're opening this issue early so the community can share feedback or raise concerns before the change lands.

## Motivation

Android 5.x–7.x devices represent a small and declining share of the active install base. Internal telemetry from Sentry's own data shows that the vast majority of SDK consumers already target API 26+ in practice.

Raising the floor to 26 lets us:

- **Reduce testing and maintenance surface** — dropping five API levels from our test matrix lets us focus CI and development effort on the versions our users actually run.
- **Remove version-check workarounds** — lets us remove compatibility code paths for pre-26 behavior.
- **Unblock future dependency upgrades** — key dependencies in the ecosystem are raising their floors (e.g. OkHttp 5.x requires API 26+). Aligning our minimum lets us adopt newer versions without holding back the entire SDK.

## Timeline

This change will ship in the **next major version** of the SDK. It will **not** affect any 8.x release. There is no fixed date yet.

## What this means for you

If your app's own `minSdkVersion` is already 26 or higher, no action is needed.

If your app still targets API 21–25, you will need to either:
1. Stay on the latest 8.x release, or
2. Raise your own `minSdkVersion` before upgrading.