bump minSdkVersion to 26 and remove old checks

[sowjanyakch]

Resolve #4369

🚧 TODO

• remove checks for android versions older than api level 26
• bump minSdkVersion to 26
• update documentation at
  • https://docs.nextcloud.com/server/latest/admin_manual/installation/system_requirements.html#mobile-apps
  • https://nextcloud-talk.readthedocs.io/en/latest/user-requirements/#mobile-apps

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

[github-actions]

Codacy

Lint

Type	master	PR
Warnings	94	157
Errors	132	81

SpotBugs

Category	Base	New
Bad practice	6	6
Correctness	16	16
Dodgy code	74	74
Internationalization	3	3
Malicious code vulnerability	3	3
Performance	5	5
Security	1	1
Total	108	108

Lint increased!

[mahibi]

added 2 minor commits.

documentation PRs:

nextcloud/spreed#13827
nextcloud/documentation#12385

[github-actions]

APK file: https://www.kaminsky.me/nc-dev/android-artifacts/4470-talk.apk



To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud Talk app.