Delete all branches before 1.6.0

[Andrew Dey]

The repo needs a clean-up and re-structuring, as part of a bigger revamp of the engine's repo maintenance.

• merge slf4j merged #127
• merge 1.6.0-display merged #124
• merge version-system disregarded as moved from 1.6.0 to later release #123
• merge fix-packages merged #134
• delete old branches (audio, examples, etc)
• handle dependabot (#144) branches
• delete physics branch until a later time