docs: Update references to Go version from 1.18 to 1.18.3

[voisey]

Summary

Note: To merge alongside #13046

Updates the Prerequisites page so that users download the correct Go version.

Test Plan

• cd magma/docs && make precommit

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[Neudrino]

Note: To merge alongside #13046

Ok for now, but usually this should be cherry picked in #13046 than, such that no coordinated merges are necessary.

[voisey]

Note: To merge alongside #13046

Ok for now, but usually this should be cherry picked in #13046 than, such that no coordinated merges are necessary.

True! I considered this once I had made the PR. I'm also happy for it to be cherry picked if preferable. cc @sebathomas