Add release notes for transpiler passes with control-flow support

[jakelishman]

As part of our larger move towards more dynamic circuits, Erick (@ewinston) has been upgrading various transpiler passes to support the control-flow operations IfElseBlock, ForLoopOp and WhileLoopOp. These features aren't completely or stable yet, so we're not publicising them in the release notes in 0.21. This issue is a tracking issue for the next milestone, to remind us to write and publish them.

See:

• Support control flow in Unroller transpiler pass #7768
• support control flow in BasisTranslator pass #7808
• add controlflow to basic_swap router #8193