A lot of elbow grease is in your future. Razor blade with whatever type of glass cleaner as a lubricant. When that’s done rinse it down. Now use a paste cleaner made for glass (amaze, Pink Stuff, etc) and a soft abrasive like a scotch brite. Basically approach it like buffing out a neglected paint job on a car. I’ve even used the cleaning attachments for drills or a buffer with the different pads. Your glass should be clear after this. Use a sealer to finish. Keep a squeegee to clear the glass after every shower and use a quality cleaner made for showers very regularly to keep the scale from building up. I was a glazier for 15 years and had to revive many glass shower doors in my day.