bump prepare-glad image to golang 1.21 to fix build

Go version using patch number was apparently not supported before go 1.21, making go build command failing with our golang:1.20-alpine image.

>go build
go: errors parsing go.mod:
/go/vuln-list-update/go.mod:3: invalid go version '1.21.5': must match format 1.23

This MR uses golang:1.21-alpine instead which supports it and fixes the build.

Relevant links

• Go 1.21 release notes

  Go 1.21 introduces a small change to the numbering of releases. In the past, we used Go 1.N to refer to both the overall Go language version and release family as well as the first release in that family.

• Go Toolchains

  Before Go 1.21, the initial release of a Go toolchain was version 1.N, not 1.N.0, so for N < 21, the ordering is adjusted to place 1.N after the release candidates.