Update C4-PlantUML to v2.8.0 (and fix missing <C4/C4_Sequence> from v2.7.0)

[kirchsth]

new C4-PlantUML v2.8.0 is released, please update to latest version.

The new main features is

• All element and boundary specific tags support $borderStyle and $borderThickness arguments
  (like the relationships with $lineStyle and $lineThickness)

UpdateElementStyle(...,?borderStyle, ?borderThickness)
AddElementTag(...,?borderStyle, ?borderThickness)
UpdateBoundaryStyle(...,?borderStyle, ?borderThickness)
AddBoundaryTag(...,?borderStyle, ?borderThickness)

Additional fix: add missing C4_Sequence.puml

• in last PR Update C4-PlantUML to v2.7.0 (Sequence Diagram support) #81 the C4_Sequence.puml was missing (!include <C4/C4_Sequence> was not working), this is fixed in this PR too

Thank you and best regards
Helmut