Upgrade to go 1.25 and minor updates

[matoszz]

I'm not 100% sure if you can have a matrix-based pipeline testing a downgraded version (vs. having a go.mod with 1.24 and testing up to 1.25) but giving it a try!

The other changes in this PR:

• Tests were throwing warnings due to non-constant format string in call to fmt.Fprintf, so the statements were changed to simply fmt.Fprint
• In contacts.go, I updated the nil check to just do len() > 0 because len() for nil maps is defined as zero (so the check was redundant)
• broadcasts.go had a malformed struct tag with an extra quote, so removed it (this would have been a bug / actual problem for anyone trying to use this)

---

Summary by cubic

Upgrade project to Go 1.25.5, update CI to test Go 1.24 and 1.25, and align Docker and dependencies. Also fix test warnings and a malformed struct tag.

• Dependencies

  • Set go directive to 1.25.5 and update Docker base to golang:1.25.
  • Update GitHub Actions to checkout/setup-go v6 and test Go 1.24/1.25.
  • Bump testify to v1.11.1.

• Bug Fixes

  • Replace fmt.Fprintf with fmt.Fprint in tests to remove non-constant format warnings.
  • Fix malformed JSON struct tag on CreateBroadcastRequest.Name.
  • Simplify UpdateContactRequest properties check to use len(r.Properties) > 0.

^{Written for commit 22707e5. Summary will update automatically on new commits.}

[cubic-dev-ai]

No issues found across 12 files

[drish]

We can keep 1.25 here

[drish]

Since 1.24 and 1.25 are the latest two stable versions, we will need to have the minimum stable version (1.24) here in the go.mod file, and keep 1.25 as part of our testing matrix, this way we make sure we're properly supporting both latest stables.

[drish]

hey @matoszz, thanks for the contribution again! left one comment related to the version on the mod file, the other changes are good.