build(deps): update specutils requirement from >=1.18.0 to >=2.1.0

[dependabot]

Updates the requirements on specutils to permit the latest version.

Release notes

Sourced from specutils's releases.

v2.1.0

New Features

• Add new flux_col keyword argument to JWST x1d reader to override flux column auto-detected from source type column (point vs surface brightness). #1244

• The readers can now read IRAF MULTISPEC files with linear and log-linear wavelength solutions; at the same time the IRAF equispec format gains the ability to read log-linear wavelength solutions. #1254

Bug Fixes

• Fixed/updated SDSS-V mwm data loader to account for a subset of targets with bad datasums. #1253

Other Changes and Additions

• Updated the JWST data readers to accomodate new WFSS file formats with SpectrumList.read(). #1252

Changelog

Sourced from specutils's changelog.

2.1.0 (2025-07-30)

New Features ^^^^^^^^^^^^

• Add new flux_col keyword argument to JWST x1d reader to override flux column auto-detected from source type column (point vs surface brightness). #1244

• The readers can now read IRAF MULTISPEC files with linear and log-linear wavelength solutions; at the same time the IRAF equispec format gains the ability to read log-linear wavelength solutions. #1254

Bug Fixes ^^^^^^^^^

• Fixed/updated SDSS-V mwm data loader to account for a subset of targets with bad datasums. #1253

Other Changes and Additions ^^^^^^^^^^^^^^^^^^^^^^^^^^^

• Updated the JWST data readers to accomodate new WFSS file formats with SpectrumList.read(). #1252

2.0.0 (2025-06-12)

New Features ^^^^^^^^^^^^

• Spectral axis can now be any axis, rather than being forced to be last. See docs for more details. [#1033, #1226]

• Spectrum now properly handles GWCS input for wcs attribute. #1074

• JWST reader no longer transposes the input data cube for 3D data and retains full GWCS information (including spatial). #1074

• Spectrum1D renamed to Spectrum. #1126

Other Changes and Additions ^^^^^^^^^^^^^^^^^^^^^^^^^^^

• Initializing a Spectrum with only a spectral axis (not full WCS) will now result in a GWCS matching the dimensionality of the flux array, rather than a 1D spectral GWCS in all cases. [#1211, #1222]

• Spectrum arithmetic now checks whether the spectral axes of the two operand Spectrum objects are equal, and fails if they are not. #1211

... (truncated)

Commits

• 8939c2b Update changelog for 2.1 release
• f63271c Merge pull request #1253 from havok2063/sdssmwm_fix
• 66bc8c3 fixing code style issues
• 2eb1119 updating changelog
• 2c8985b fix for mwmstar files with bad datasums
• 2471077 fixing the test for windows
• 52c0a5d fixing code style issues
• 310b38c updating changelog
• 2e25078 fix for mwmstar files with bad datasums
• 64ed60a Merge pull request #1254 from hamogu/io_multispec_linear
• Additional commits viewable in compare view

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