Update builds to libvips 8.15.1

[ramses-lopez]

Running the build script with VIPS_VERSION=8.15.1 ./build.sh was failing due to this error:

2.441 The Meson build system
2.441 Version: 1.3.1
2.441 Source dir: /usr/local/src/libvips-8.15.1
2.441 Build dir: /usr/local/src/libvips-8.15.1/build
2.441 Build type: native build
2.441
2.441 meson.build:1:0: ERROR: Value "false" (of type "string") for combo option "Build GObject introspection data" is not one of the choices. Possible choices are (as string): "enabled", "disabled", "auto".

This PR updates the introspection flag from false to disabled on Dockerfile.heroku-20 and Dockerfile.heroku-22

Fixes #46

[ramses-lopez]

@brandoncc I ran into this error on my rails application. I was getting errors when users uploaded *.jfif files. This was fixed by the libvips maintainer on 8.15.1

[ryanstrickler]

@ramses-lopez I've been getting the same error. I'm going to try using your fork and see if that fixes it.

[ramses-lopez]

@ryanstrickler sounds good -- I've been using for the past two weeks without any issues