Rebrand stable9 to Nextcloud

[MariusBluem]

• Rebrand README.md
• Rebrand issue_template.md

@MorrisJobke Do we have badges (build, ...) for stable9?

[MorrisJobke]

@MorrisJobke Do we have badges (build, ...) for stable9?

Not yet. We only have this information per PR and not per branch.

[MorrisJobke]

Fix is in #518

[nickvergessen]

Merged