DEPS: Upgrade shoutrrr dependency and remove Go toolchain pinning (#3858)

- Follow up for
https://github.com/StackExchange/dnscontrol/pull/3838#discussion_r2539801899

github.com/nicholas-fedor/shoutrrr < 0.12.1 was pinning the toolchain.
Which in turn required dnscontrol to pin it as well.

This has been fixed upstream via

- https://github.com/nicholas-fedor/shoutrrr/pull/464

and this PR is adopting the [following upstream
release](https://github.com/nicholas-fedor/shoutrrr/releases/tag/v0.12.1).

@gvengel Could you help with testing this PR? Thanks in advance!


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Co-authored-by: Tom Limoncelli <6293917+tlimoncelli@users.noreply.github.com>

go.mod

@@ -1,6 +1,6 @@
 module github.com/StackExchange/dnscontrol/v4
 
-go 1.25.3
+go 1.25
 
 retract v4.8.0