Bump version to 1.6.2 and relax certbot upper bound (<6)

[TheophileDiot]

This pull request performs a minor version bump and updates the certbot dependency constraint to allow newer major releases.

Changes

1. Version bump

   • __version__ updated from 1.6.1 → 1.6.2
   • Reflects a dependency constraint update only (no functional changes).

2. Dependency update

   • Updated certbot requirement:

     • From: certbot >= 2.4.0, <4
     • To: certbot >= 2.4.0, <6

   • This expands compatibility to include certbot 4.x and 5.x releases while preserving the existing minimum supported version.

Rationale

• Recent certbot major releases remain compatible with the plugin.
• The previous <4 upper bound unnecessarily restricted installations in environments using newer certbot versions.
• No breaking changes were identified that would require additional code modifications.

Impact

• No runtime behavior changes.
• No API changes.
• Dependency compatibility broadened.
• Patch-level release (1.6.2) is appropriate as functionality remains unchanged.