Fix non-constant format string error on go 1.26

[dsimansk]

Changes

• Fix non-constant format string error on go 1.26

Bumping go directive in go.mod to force error fix verification through unit tests. The 1.26's go vet is has tighter recognition of string formatters not being present.

testing/duck/testbindable.go:153:63: non-constant format string in call to (knative.dev/pkg/apis.ConditionManager).MarkFalse

Appeared here: #3391

Release Note

Fix non-constant format string error on go 1.26

/cc @gauron99 @kahirokunn

[knative-prow]

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[dsimansk]

There's couple more occasions of same use across our repos: #3387

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.88%. Comparing base (d2a153a) to head (3ad1cca).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3392      +/-   ##
==========================================
+ Coverage   74.85%   74.88%   +0.02%     
==========================================
  Files         189      189              
  Lines        8349     8349              
==========================================
+ Hits         6250     6252       +2     
+ Misses       1854     1853       -1     
+ Partials      245      244       -1     

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