Upgrade pdfcpu dependency to 0.11.0

[tgvaughan]

Hi @jo3-l, thanks for the useful tool! I know that much of the functionality has been subsumed into pdfcpu itself these days, but I still very much prefer using your bookmark format to JSON.

This pull request simply upgrades the version of the pdfcpu dependency to 0.11.0, requiring a 1-line change to accommodate a change in the pdfcpu bookmarks API. Without this upgrade, markpdf produces the following error when applied to some PDF files:

markpdf: could not extract pdf info: dict=StructElementDict entry=E: unsupported in version 1.4
This file could be PDF/A compliant but pdfcpu only supports versions <= PDF V1.7

With the upgrade in place, markpdf again works.

[jo3-l]

Thanks, glad to hear the tool was of use to you!

[jo3-l]

Hm, CI looks like it's failing on the go 1.23.0 line in go.mod. I'll take a look to see if this is an issue with the testing setup later if you don't get to it first.

[tgvaughan]

Seems one of the module updates required go 1.23 - upgrading the version used in the CI test looks like it resolves the problem. (I'm not a go programmer though so beware! :) )

[jo3-l]

Looks good. Thanks again!