CompatHelper: bump compat for "ArchGDAL" to "0.8"

[github-actions]

This pull request changes the compat entry for the ArchGDAL package from 0.6, 0.7 to 0.6, 0.7, 0.8.

This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[coveralls]

Pull Request Test Coverage Report for Build 2183363921

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.4%

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Totals
Change from base Build 2173014894:	0.0%
Covered Lines:	1686
Relevant Lines:	2578

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💛 - Coveralls

[codecov]

Codecov Report

Merging #87 (7ed68d9) into main (f5facf0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #87   +/-   ##
=======================================
  Coverage   73.25%   73.25%           
=======================================
  Files          26       26           
  Lines        2004     2004           
=======================================
  Hits         1468     1468           
  Misses        536      536           

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