fix(deps): update module golang.org/x/mod to v0.29.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
golang.org/x/mod	v0.28.0 -> v0.29.0

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[renovate]

⚠️ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

• any of the package files in this branch needs updating, or
• the branch becomes conflicted, or
• you click the rebase/retry checkbox if found above, or
• you rename this PR's title to start with "rebase!" to trigger it manually

The artifact failure details are included below:

File name: build/go.sum

Command failed: go get -t ./...
go: downloading golang.org/x/mod v0.29.0
go: go.flipt.io/build imports
	go.flipt.io/build/internal/dagger: cannot find module providing package go.flipt.io/build/internal/dagger

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.87%. Comparing base (33aaef0) to head (75a0da0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4873   +/-   ##
=======================================
  Coverage   63.87%   63.87%           
=======================================
  Files         172      172           
  Lines       17662    17662           
=======================================
  Hits        11282    11282           
  Misses       5705     5705           
  Partials      675      675           

Flag	Coverage Δ
unittests	63.87% <ø> (ø)

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