action-activity-report: drop dead label aliases and settle discuss / low-priority handling

[mmcky]

tools/highlights.py in action-activity-report reads and writes label names the standard does not define, and carries a note that is now false. From QuantEcon/qeps#3.

Problem	Fix
Dead aliases feature, priority: high, chore, docs	Remove — they map to enhancement, high-priority, maintenance and documentation, and the migration renames them out of existence
A comment asserting that infrastructure no longer exists	Correct it — infrastructure is in the standard, with a description 98 characters long
discuss and low-priority handling	Settle it — low-priority is standard, discuss is not, and the report ranks by label

This emitter differs from the other two: it reads labels in order to rank and surface work, so a dead alias here is silent under-reporting rather than a wrong tag. It is also the emitter whose output people read every week.

This depends on the software widening batch, and is verified by qe gh bots check (QuantEcon/cli#5).

Acceptance

• No label name the standard does not define is read or written
• The infrastructure note is corrected
• discuss and low-priority handling is decided and recorded here
• One weekly report is produced and read after the change, to confirm nothing dropped out of the rankings