Bump go version to 1.26.3 to fix CVEs

[ianratcliffe]

The golang.org/x/net library is impacted by the following CVE's:
CVE-2026-39821
CVE-2026-33814
CVE-2026-39824
CVE-2026-27136
CVE-2026-25681
CVE-2026-42506
CVE-2026-42502
CVE-2026-25680

[newrelic-php-agent-bot]

Thank you for your issue report. A member of the New Relic PHP Agent Team will review your report and endeavor to respond in a timely manner.
Need troubleshooting help? Please review some of our common troubleshooting documentation, starting with generating debug logs. From there, additional resources and tips are available in our troubleshooting section, accessible from the left-hand panel of our documentation site. Additionally, please be sure to visit our Explorer's Hub, where other members of the community may have solved an issue similar to yours with the help of our Support team members.
Have a question regarding support for an Operating System, PHP Version, Framework, or Library? Please take a look at our Compatibility Docs.
Binary support for ARM-based CPU architecture is now available via the tarball installation method for PHP Versions 8.0+!
General installation instructions can be found on our Installation Overview page.
Need more information about an Agent INI setting? Our Configuration page contains an explanation for each INI setting and how to modify the defaults.
Looking for something else? All New Relic PHP Agent Documentation can be found under our Introduction page.

[zsistla]

Please note the golang version has been updated to 1.26.3 since May 12 in release 12.7.0.36
https://docs.newrelic.com/docs/release-notes/agent-release-notes/php-release-notes/php-agent-12-7-0-36/