update go version to fix CVE-2023-45284,CVE-2023-39326,CVE-2023-48795

[kschoche]

docker scout reported a few MEDIUM/LOW CVEs for 0.4.2, updating to go 1.21.5 resolves 3 of them:

   0C     0H     2M     0L  stdlib 1.21.3 pkg:golang/stdlib@1.21.3
    ✗ MEDIUM CVE-2023-45284
      https://scout.docker.com/v/CVE-2023-45284
      Affected range : >=1.21.0-0
                     : <1.21.4
      Fixed version  : 1.21.4

    ✗ MEDIUM CVE-2023-39326
      https://scout.docker.com/v/CVE-2023-39326
      Affected range : >=1.21.0-0
                     : <1.21.5
      Fixed version  : 1.21.5

   0C     0H     1M     0L  golang.org/x/crypto 0.15.0 pkg:golang/golang.org/x/crypto@0.15.0

    ✗ MEDIUM CVE-2023-48795 [Insufficient Verification of Data Authenticity]
      https://scout.docker.com/v/CVE-2023-48795
      Affected range : <0.17.0
      Fixed version  : 0.17.0
      CVSS Score     : 5.9
      CVSS Vector    : CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N