meson: bump major version

Because this is actually a rather large release, I kinda feel it's about
time to bump the major version. I still haven't decided if I want to
switch to semver or not to avoid spilling into the three digits, but oh
well. Now it's done, anyways.
11 jobs for v3.104.0-rc1 in 9 minutes and 29 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #499181
aarch64 docker
build-aarch64

00:00:20

passed #499182
macos
build-macos

00:00:05

passed #499177
amd64 docker
build-ubuntu

00:00:41

passed #499178
amd64 docker
build-ubuntu-static

00:00:57

passed #499179
amd64 docker
build-win32

00:00:16

passed #499180
amd64 docker
build-win64

00:00:09

 
  Test
passed #499184
gpu
test-gpu

00:01:52

76.0376%
passed #499185
gpu
test-gpu-asan

00:02:17

passed #499186
gpu
test-gpu-msan

00:01:57

passed #499187
gpu
test-gpu-ubsan

00:01:57

passed #499183
amd64 docker
test-ubuntu-scan

00:00:24