Change server-tokens default value to false

[aledbf]

What this PR does / why we need it:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Which issue/s this PR fixes

fixes #6158

How Has This Been Tested?

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

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[billinghamj]

Just to flag, this seems to break the allow-backend-server-header config field - preventing it from being used by itself. Now, due to the more_clear_headers directive, it only works if you also set server-tokens to true.