Pin syn 2.0.106 in MSRV CI

[psychon]

See dtolnay/syn@9dbb5b7

Sadly, we have two versions of syn in our dependency tree. Thus, cargo
requires us to specify the exact version of syn that we want to
downgrade. This in turn means that this pin will break on every new syn
release. :-(

[psychon]

Previous none-working attempts:

https://github.com/psychon/x11rb/actions/runs/18993875408/job/54250880768

Run cargo update --package syn --precise 2.0.106
error: There are multiple `syn` packages in your project, and the specification `syn` is ambiguous.
Please re-run this command with `-p <spec>` where `<spec>` is one of the following:
  syn@1.0.109
  syn@2.0.108

https://github.com/psychon/x11rb/actions/runs/18994178740/job/54251604735

Run cargo update --package syn@2 --precise 2.0.106
error: invalid package ID specification: `syn@2`

	Did you mean `syn`?

Caused by:
  cannot parse '2' as a semver

https://github.com/psychon/x11rb/actions/runs/18994247960/job/54251795173

Run cargo update --package syn@2.0 --precise 2.0.106
error: invalid package ID specification: `syn@2.0`

Caused by:
  cannot parse '2.0' as a semver

[eduardosm]

With Rust 1.74 or newer, it is possible to use --package syn@2. Is there anything preventing to bump our MSRV? Debian is at 1.85.

[psychon]

Dunno.

I took a look at a random selection of reverse dependencies. arboard has an MSRV of 1.71 Same for softbuffer. xkeysym is even at 1.58.1 and depends on an older version of x11rb. global-hotkey has a MSRV of 1.77 and vulkano uses 1.84.

But syn does frequent enough releases that the pinning I propose in this PR does not really work.

I guess bumping the MSRV to 1.68 is the best course of action. That's syn's MSRV and that removes the need for pinning an older version in CI.

[psychon]

Alternatively, we could somehow exclude the cairo-example from the MSRV build. Only that depends on syn. 🙈