Make page_above/below strings rather than ints

[ml-evs]

This looks like a simple oversight on our side when we first created these models.

page_below and page_above should instead refer to the ID values that the query should be performed relative to.

Annoyingly this PR also reorders some fields in the OpenAPI spec that keep getting jumbled (see #1429). I think this PR at least fixes them back to being alphabetical, but we should make sure we don't accept PRs that reorder this in future.

[codecov]

Codecov Report

Merging #1529 (af0036a) into master (c3ed95d) will not change coverage.
The diff coverage is n/a.

❗ Current head af0036a differs from pull request most recent head fd64cfd. Consider uploading reports for the commit fd64cfd to get more accurate results

@@           Coverage Diff           @@
##           master    #1529   +/-   ##
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  Coverage   91.10%   91.10%           
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  Files          74       74           
  Lines        4519     4519           
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  Hits         4117     4117           
  Misses        402      402           

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project	91.10% <ø> (ø)
validator	90.99% <ø> (ø)

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Impacted Files	Coverage Δ
optimade/server/query_params.py	98.27% <ø> (ø)

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[ml-evs]

Might still want to adjust the default values here

[JPBergsma]

I think there are further references to page_above and page_below on lines 146 to 151 in query_params.py These should be updated as well.