TimelockController: Use default admin role

[frangio]

TimelockController currently uses a custom non-zero TIMELOCK_ADMIN role that has complicated the introduction of the new CANCELLER_ROLE in 4.6 and necessitated a custom migration to set the role admin that would not have been necessary had DEFAULT_ADMIN_ROLE been used.

Moving forward we should always use the default admin role rather than custom roles.

[Niceural]

Hi! Does this issue need a pr?

[frangio]

It's a breaking change so we're not doing it yet.

[frangio]

Fixed by #3799 for future 5.0 release.