Thoma Bravo has agreed to buy Dayforce, a Minneapolis-based HR software provider, for $12.3 billion.Why it matters: This comes against the backdrop of a weakening U.S. labor market, which could set the stage for additional HR tech mergers.Zoom in: Thoma Bravo will pay $70 per share, representing a 32% premium to Dayforce shares before media reports of a possible deal.Abu Dhabi Investment Authority will provide a "significant minority investment."Catch up quick: Dayforce previously was known as C...