docs(UPGRADE.md) add note to PR template

[michaelbeaumont]

Summary

This document should be kept up to date rather than updated before a release.

[codecov-commenter]

Codecov Report

Merging #3358 (225ca08) into master (9a7dc1e) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3358      +/-   ##
==========================================
- Coverage   52.25%   52.24%   -0.01%     
==========================================
  Files         929      929              
  Lines       54421    54421              
==========================================
- Hits        28437    28433       -4     
- Misses      23758    23760       +2     
- Partials     2226     2228       +2     

Impacted Files	Coverage Δ
pkg/plugins/leader/postgres/leader_elector.go	93.61% <0.00%> (-6.39%)	⬇️
pkg/plugins/runtime/gateway/route/sorter.go	61.53% <0.00%> (-5.13%)	⬇️
pkg/xds/generator/direct_access_proxy_generator.go	83.90% <0.00%> (+1.14%)	⬆️

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[lahabana]

Please fix the format of the PR btw