Rename 'update' command to 'upgrade'

[JaredHatfield]

Summary

Renames the update CLI command to upgrade across the entire codebase.

Changes

• CLI command: gogitup update → gogitup upgrade
• update.go / update_test.go replaced with upgrade.go / upgrade_test.go
• All message strings updated ("Updating" → "Upgrading", "Updated" → "Upgraded", "Failed to update" → "Failed to upgrade")
• All documentation updated to reflect the new command name

Rationale

• upgrade is more semantically accurate — the command installs newer versions, not just checks for them
• Aligns with conventions in other package managers (apt upgrade, brew upgrade, npm upgrade)
• No logic changes, tests map 1:1