Update documentation to reflect v6 and v7 branches are no longer maintained

[anujc25]

Issue Type: Documentation Update

Summary

The CF CLI documentation needs to be updated to clearly reflect that v6 and v7 branches are no longer actively maintained, and users should migrate to v8.

Current State

Currently, the documentation and issue templates still reference v6 and v7 as if they are actively supported:

1. README.md (lines 41-43) states: "There are now three supported versions of the cf CLI" and lists v7 and v8 as supported
2. Bug report template includes detailed v6 and v7 support information
3. Feature request template (line 34) mentions v6 line is "no longer under active development" but doesn't clarify v7 status
4. Installation guides still exist for both v6 and v7 versions

Proposed Changes

1. Update README.md

• Remove references to v6 and v7 as "supported versions"
• Add clear deprecation notice for v6 and v7
• Update the "Getting Started" section to focus on v8
• Add migration guidance from v6/v7 to v8

2. Update Issue Templates

• Bug Report Template: Add clear notice that v6 and v7 are no longer maintained
• Feature Request Template: Update the note to clarify both v6 and v7 status
• Consider adding a separate template for v6/v7 legacy issues (if any are still accepted)

3. Update Installation Documentation

• Add deprecation warnings to v6 and v7 installation guides
• Redirect users to v8 installation guide
• Consider archiving or moving legacy installation docs

4. Update Other Documentation

• Review and update any ADRs (Architecture Decision Records) that reference v6/v7 support
• Update contributing guidelines if they reference v6/v7 development
• Update any wiki pages or external documentation links

Additional Context

Based on the current codebase:

• The feature request template already mentions v6 is no longer under active development
• The bug report template still provides detailed v6/v7 support information
• The README suggests three versions are supported when only v8 is actively maintained

This documentation update will help:

• Set clear expectations for users
• Reduce confusion about which versions receive updates
• Guide users toward the actively maintained v8 CLI
• Reduce maintenance burden on legacy version documentation

Priority

Medium - This is important for user clarity and project maintenance, but not blocking any current functionality.

[moleske]

we should also close #3515 since I think this issue and that are duplicates