feat: Add optional fastBurn/slowBurn aliases for page and ticket alerts

[cxdy]

Closes #833

Accept fast_burn/slow_burn as non-breaking aliases of page_alert/ticket_alert and the CRD pageAlert/ticketAlert keys. Old names still work; setting both for one class is an error. Generated rules stay page/ticket.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.46939% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.70%. Comparing base (8a3be4f) to head (80b6dfd).

Files with missing lines	Patch %	Lines
pkg/kubernetes/api/sloth/v1/alerting.go	77.77%	4 Missing and 4 partials ⚠️
pkg/prometheus/api/v1/alerting.go	77.77%	4 Missing and 4 partials ⚠️
internal/storage/io/k8s_sloth.go	66.66%	2 Missing and 2 partials ⚠️
internal/storage/io/sloth.go	66.66%	2 Missing and 2 partials ⚠️
...g/kubernetes/api/sloth/v1/zz_generated.deepcopy.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #843      +/-   ##
==========================================
+ Coverage   31.30%   31.70%   +0.39%     
==========================================
  Files         149      151       +2     
  Lines        9228     9314      +86     
==========================================
+ Hits         2889     2953      +64     
- Misses       6088     6100      +12     
- Partials      251      261      +10     

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[wbollock]

I prefer fast_burn and slow_burn compared to high and low. high and low makes it feel like they are related to severities or urgency rather than burn rate