Update MAR_CHANNEL_ID for the alpha

Currently the alpha and stable release use the same channel id.

In legacy/trac#32498 (moved) we changed ACCEPTED_MAR_CHANNEL_IDS for the alpha to torbrowser-torproject-alpha,torbrowser-torproject-release so that new alpha releases can be updated using the torbrowser-torproject-alpha channel id.

When enough time has passed and we think that most people have updated to a release that supports this new channel id, we should change MAR_CHANNEL_ID for the alpha.