fix: update Go version to 1.26 and dependencies for security improvements

[t4niwa]

Description

Bump golang.org/x/net from v0.23.0 to v0.55.0 to fix a known vulnerability.

Since the newer x/net needs a recent toolchain, I also bumped Go 1.23 to 1.26.

While at it I fixed a go vet warning in third_party/webhook/authz.go.

No behavior change.

Nothing in garm itself changed functionally. go build and go test -race ./... pass.

Assignees

• Assignees is set

Type of changes

• dependencies
• bug (go vet fix)

Flags

• Breaks backward compatibility
• Requires a documentation update
• Has untestable code

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Checklist

• Followed the guidelines in the CONTRIBUTING document
• Added prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip] in the PR title if necessary
• Tested and linted the code
• Commented the code
• Made corresponding changes to the documentation

Checklist for maintainer

• Use Squash and merge
• Double-confirm the merge message has prefix ...
• Delete the branch after merge