fix: increase max XML size to 1.5GB

[xbhouse]

Some repos (RHEL 9.8 EUS/E4S aarch64 baseos) exceed 1GB. This increases the max XML size yummy can parse.

Testing steps:

1. Use your local yummy directory as the backend yummy package in content-sources-backend/go.mod and run go mod tidy:

replace [github.com/content-services/yummy](http://github.com/content-services/yummy) => /path/to/yummy

2. Try to introspect a large repo (https://cdn.redhat.com/content/e4s/rhel9/9.8/aarch64/baseos/os/ or https://cdn.redhat.com/content/eus/rhel9/9.8/aarch64/baseos/os/). It shouldn't fail.

3. Without this PR, you'll see an error like this:

An introspection error occurred: XML syntax error on line 12654284: unexpected EOF