[Snyk] Security upgrade golang from 1.20.13 to 1.21

[mefellows]

This PR was automatically created by Snyk using the credentials of a real user.

Keeping your Docker base image up-to-date means you’ll benefit from security fixes in the latest version of your chosen image. #### Changes included in this PR - Dockerfile We recommend upgrading to `golang:1.21`, as this image has only 78 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected. Some of the most important vulnerabilities in your base image include: | Severity | Priority Score / 1000 | Issue | Exploit Maturity | | :------: | :-------------------- | :---- | :--------------- | |  | **614** | Improper Verification of Cryptographic Signature
[SNYK-DEBIAN12-GNUTLS28-6159410](https://snyk.io/vuln/SNYK-DEBIAN12-GNUTLS28-6159410) | No Known Exploit | |  | **614** | Information Exposure
[SNYK-DEBIAN12-GNUTLS28-6159418](https://snyk.io/vuln/SNYK-DEBIAN12-GNUTLS28-6159418) | No Known Exploit | |  | **514** | CVE-2023-7008
[SNYK-DEBIAN12-SYSTEMD-6137714](https://snyk.io/vuln/SNYK-DEBIAN12-SYSTEMD-6137714) | No Known Exploit | |  | **514** | CVE-2023-7008
[SNYK-DEBIAN12-SYSTEMD-6137714](https://snyk.io/vuln/SNYK-DEBIAN12-SYSTEMD-6137714) | No Known Exploit | |  | **514** | Out-of-bounds Read
[SNYK-DEBIAN12-TAR-3253526](https://snyk.io/vuln/SNYK-DEBIAN12-TAR-3253526) | No Known Exploit | --- **Note:** _You are seeing this because you or someone else with access to this repository has authorized Snyk to open fix PRs._ For more information: 🧐 [View latest project report](https://app.snyk.io/org/pact-foundation-808/project/0c88a376-aac5-48c1-baa5-58b7ef92b022?utm_source=github&utm_medium=referral&page=fix-pr) 🛠 [Adjust project settings](https://app.snyk.io/org/pact-foundation-808/project/0c88a376-aac5-48c1-baa5-58b7ef92b022?utm_source=github&utm_medium=referral&page=fix-pr/settings) [//]: # 'snyk:metadata:{"prId":"7e2869b0-c777-496d-b44f-f78afb273a95","prPublicId":"7e2869b0-c777-496d-b44f-f78afb273a95","dependencies":[{"name":"golang","from":"1.20.13","to":"1.21"}],"packageManager":"dockerfile","projectPublicId":"0c88a376-aac5-48c1-baa5-58b7ef92b022","projectUrl":"https://app.snyk.io/org/pact-foundation-808/project/0c88a376-aac5-48c1-baa5-58b7ef92b022?utm_source=github&utm_medium=referral&page=fix-pr","type":"auto","patch":[],"vulns":["SNYK-DEBIAN12-GNUTLS28-6159410","SNYK-DEBIAN12-GNUTLS28-6159418","SNYK-DEBIAN12-SYSTEMD-6137714","SNYK-DEBIAN12-TAR-3253526"],"upgrade":["SNYK-DEBIAN12-GNUTLS28-6159410","SNYK-DEBIAN12-GNUTLS28-6159418","SNYK-DEBIAN12-SYSTEMD-6137714","SNYK-DEBIAN12-SYSTEMD-6137714","SNYK-DEBIAN12-TAR-3253526"],"isBreakingChange":false,"env":"prod","prType":"fix","templateVariants":["updated-fix-title","priorityScore"],"priorityScoreList":[614,614,514,514],"remediationStrategy":"vuln"}' --- **Learn how to fix vulnerabilities with free interactive lessons:** 🦉 [Learn about vulnerability in an interactive lesson of Snyk Learn.](https://learn.snyk.io/?loc=fix-pr)

[coveralls]

coverage: 37.291%. first build when pulling f951fd7c2f29758f2cdcd4969f0bf17fd9db813a on snyk-fix-131b3d296d81afa2e18a5b60e18c9aca into 901aea82975ef17de71866bef6d017f6c4d55d09 on master.