Update WCSLIB to version 8.1

[mcara]

Description

This PR updates built-in WCSLIB distributed with astropy. Excerpts from WCSLIB's changelog:

WCSLIB version 8.1 (2023/07/06)
-------------------------------

Changes in the contents of the auxprm struct (in 8.0 beta) necessitated
incrementing the major version number from 7 to 8, i.e. the ABI changed.
Took the opportunity for minor tweaks to wcsprm and disprm.

* C library

  - In disprm, changed the order of maxdis and totdis to ensure correct
    alignment of doubles on 32-bit machines.

WCSLIB version 8.0 beta (2023/07/01)
------------------------------------

Beta test version, not publically released.

* C library

  - Support planetary keywords A_RADIUS, B_RADIUS, C_RADIUS, BLON_OBS,
    BLAT_OBS, and BDIS_OBS in auxprm by analogy with the Solar keywords
    added at 7.1.  Requested by Chiara Marmo (Observatoire de Paris).

  - Added wcsprm::time to the wcsprm struct to record the TIME axis
    number (along with lng, lat, and spec).

  - Fixed a bug in wcspih() and wcsbth() where trailing blanks were not
    stripped from string keyvalues.  Reported by Naveen Dukiya.

  - Fixed a problem affecting thread safety in disp2x(), disx2p(), and
    diswarp().  Reported by Mohammad Akhlaghi.

For a full list of changes, see WCSLIB's CHANGELOG
or astropy/cextern/wcslib/CHANGES

This update is to support work on #14820.

Many thanks to @cmarmo !

[github-actions]

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[nden]

LGTM

[pllim]

Mihai and I discussed the milestone offline. We agreed now that this does not need to be backported, so it will land in v6.0. Thanks, all!

[pllim]

@cmarmo , you can pick this up now if you rebase. 😸

[pllim]

FYI, I had to fix your change log in d59d7fa because RTD was failing with a warning:

docs/changelog.rst:5: WARNING: Duplicate explicit target name: "astropy/cextern/wcslib/changes".