Update Briar headless to 1.2.7

• https://code.briarproject.org/briar/briar/-/wikis/changelog#anchor-127
• https://code.briarproject.org/briar/briar/-/tags/release-1.2.7
• https://media.dorfbrunnen.eu/briar/jars/009/

changed milestone to %GTK 0.1.0-alpha1

added T: task label

added S: in progress label

changed title from Update Briar headless to latest release to Update Briar headless to 1.2.7

changed the description

mentioned in commit 9eb41863

mentioned in merge request !6 (merged)

added S: in code review label and removed S: in progress label

closed via merge request !6 (merged)

closed via commit 9eb41863

added R: fixed label and removed S: in code review label