chore: updates for undici watch

[cmwylie19]

Description

Undici fetch is going to be the default watch mechanism in Pepr v0.39.0. This PR updates the docs, watchConfig, and tests that references http2 and documents the PEPR_USE_LEGACY_WATCH env var.

ON HOLD - defenseunicorns/kubernetes-fluent-client#399 KFC needs this merged and released

Related Issue

Fixes #1383

Relates to #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Unit, Journey, E2E Tests, docs, adr added or updated as needed
• Contributor Guide Steps followed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (main@7020907). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1384   +/-   ##
=======================================
  Coverage        ?   78.32%           
=======================================
  Files           ?       36           
  Lines           ?     1698           
  Branches        ?      379           
=======================================
  Hits            ?     1330           
  Misses          ?      366           
  Partials        ?        2           

Files with missing lines	Coverage Δ
src/lib/watch-processor.ts	78.64% <ø> (ø)

[samayer12]

Conducted sync review. Potentially breaking change for end-users that rely on PEPR_HTTP2_WATCH, but we'll cover that in release notes.