Update go-crossplane version to 0.4.15

[dareste]

Proposed changes

This is a change to use the latest go-crossplane version in the agent (0.4.15). The version currently used (0.4.1) is detecting valid nginx configurations as wrong ones. For instance:

map "$hostname" $myvar {
    hostnames;
    host1.local 1;
    host2.local 2;
    default	3;
}

Crossplane 0.4.1 refuses this valid map with an invalid number of parameters error.

Checklist

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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