Rename "zenith" to "neon"

[andreasscherbaum]

Usually RFC documents are not modified, but the vast mentions of "zenith" in early RFC documents make it desirable to update the product name to today's name, to avoid confusion.

Problem

Early RFC documents use the old "zenith" product name a lot, which is not something everyone is aware of after the product was renamed.

Summary of changes

Replace occurrences of "zenith" with "neon".
Images are excluded.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2448 tests run: 2326 passed, 0 failed, 122 skipped (full report)

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Flaky tests (2)

Postgres 15

• test_vm_bit_clear_on_heap_lock: debug

Postgres 14

• test_delete_timeline_client_hangup: debug

Code coverage* (full report)

• functions: 28.8% (6933 of 24084 functions)
• lines: 47.4% (42568 of 89817 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
8d659ff at 2024-03-01T12:42:28.494Z :recycle:}

[andreasscherbaum]

Ok, makes sense to update the links while we are on it. I'm going to fix that (tomorrow), thanks for pointing that out.