Downgrade toolchain version

[vtopc]

On master branch the toolchain directive adds a weird side effect:

# go version
go version go1.21.12 darwin/arm64

# cd holster
# go version
go: downloading go1.22.2 (darwin/arm64)
go version go1.22.2 darwin/arm64

and our 1.21 CI tests test it on 1.22:

Run actions/setup-go@v5
Setup go version spec 1.21.x
Found in cache @ /opt/hostedtoolcache/go/1.21.10/x64
Added go to the path
Successfully set up Go version 1.21.x
go: downloading go1.22.2 (linux/amd64)
...
Cache is not found
go version go1.22.2 linux/amd64

https://github.com/mailgun/holster/actions/runs/9158167548/job/25175965954

but all other services are using 1.21

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It was added in https://github.com/mailgun/holster/pull/191/files?w=1

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What is Go Toolchains - https://go.dev/doc/toolchain

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Tested with https://github.com/mailgun/maverick-score/pull/84