Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (file-contains-trailing-whitespace)
• Bump debhelper from deprecated 7 to 12. (package-uses-deprecated-debhelper-compat-version)
• Set debhelper-compat version in Build-Depends. (uses-debhelper-compat-file)
• Fix day-of-week for changelog entry 1.2~b-2. (debian-changelog-has-wrong-day-of-week)
• Update standards version to 4.5.0, no changes needed. (out-of-date-standards-version)

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/brailleutils/96018531-de2a-48e6-b2cc-e1b96dd3d615.

These changes affect the binary packages; see the debdiff

You can also view the diffoscope diff (unfiltered).