v1.25.0: PGO, better exit codes, \u escape

This release includes several minor changes:

• Build binaries with PGO on Go 1.21. GoAWK on Go 1.21 is about 10% faster than on Go 1.20, and PGO makes it another 5-6% faster.
• Bumps up the minimum supported Go version from 1.15 to 1.16.
• Makes the return value of system() and close() for pipes more closely match Gawk's. Internal refactoring of the input and output stream implementation to make this happen. See issues #203 #204 #205 and thanks @juster!
• Support the \u Unicode string escape that's been added to onetrueawk and Gawk recently. #212