release: remove xDaysBeforeRelease, unify issue and timelines

[bobheadxi]

closes https://github.com/sourcegraph/sourcegraph/issues/17364, documented by sourcegraph/about#2355

• Remove xDaysBeforeRelease and associated features (calendar events, template args, etc)
• Unify generation of issues and calendar events, as well as things like release naming. Previously this was rather brittle to change and had a lot of code duplication
• Other improvements:
  • tracking:issues now handles all tracking issue generation
  • Calendar dry run
  • Backlinks to "parent" issue for child issues related to a release
  • Update patch request issue

Pending completion of https://github.com/sourcegraph/sourcegraph/issues/17144 - @davejrt please let this merge before doing the post-release steps!

[codecov]

Codecov Report

Merging #17401 (afe5a50) into main (749f6de) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #17401      +/-   ##
==========================================
- Coverage   51.80%   51.80%   -0.01%     
==========================================
  Files        1716     1716              
  Lines       85051    85051              
  Branches     7752     7752              
==========================================
- Hits        44064    44061       -3     
- Misses      37081    37082       +1     
- Partials     3906     3908       +2     

Flag	Coverage Δ
go	50.84% <ø> (-0.01%)	⬇️
integration	30.70% <ø> (-0.02%)	⬇️
storybook	29.93% <ø> (ø)
typescript	54.12% <ø> (-0.01%)	⬇️
unit	34.74% <ø> (ø)

Impacted Files	Coverage Δ
client/web/src/tree/TreeRoot.tsx	83.33% <0.00%> (-3.04%)	⬇️
.../internal/codeintel/resolvers/graphql/locations.go	83.50% <0.00%> (-2.07%)	⬇️
...nal/campaigns/resolvers/changeset_apply_preview.go	65.18% <0.00%> (+0.74%)	⬆️