Change show-clipboard attribute to file-handling-enabled-targets attribute

README.md

assets/dbp-signature.html.ejs

examples/dbp-official-signature-pdf-upload/index.html

examples/dbp-signature/index.html

src/dbp-official-signature-pdf-upload.js

src/dbp-official-signature-pdf-upload.metadata.json

src/dbp-qualified-signature-pdf-upload.js

src/dbp-qualified-signature-pdf-upload.metadata.json

src/dbp-signature-lit-element.js

src/dbp-signature-verification-full.js

src/dbp-signature-verification-full.metadata.json