Fix minor upgrade logic in go upgrade tool

[mhamza15]

Description

In #18402, logic was added to make sure that patch versions are considered for upgrades, but this had the unintended effect of making non-major version upgrades stop working. This fixes that and also additionally adds a stricter check for existing PRs in the workflow.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.92%. Comparing base (4a3e6a3) to head (91d6d9d).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
go/tools/go-upgrade/go-upgrade.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19176      +/-   ##
==========================================
+ Coverage   69.91%   69.92%   +0.01%     
==========================================
  Files        1613     1613              
  Lines      216122   216126       +4     
==========================================
+ Hits       151097   151126      +29     
+ Misses      65025    65000      -25     

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