Raise MSRV to 1.84 (#6848)

Prerequisite of https://github.com/emilk/egui/pull/6744.
See: https://github.com/gfx-rs/wgpu/pull/7218,
https://github.com/gfx-rs/wgpu/pull/7425

Please be aware that Rust 1.84 enables some (more) WASM extensions by
default, and ships with an `std` built with them enabled:
https://blog.rust-lang.org/2024/09/24/webassembly-targets-change-in-default-target-features/
According to `rustc +1.84 --print=cfg --target wasm32-unknown-unknown`,
these are: `multivalue`, `mutable-globals`, `reference-types`, and
`sign-ext`.
(c.f. `rustc +1.84 --print=cfg --target wasm32-unknown-unknown -C
target-cpu=mvp` enabling none.)
For reference: https://webassembly.org/features/

----

If support is desired for ancient/esoteric browsers that don't have
these implemented, there are two ways to get around this:
- Target `wasm32v1-none` instead, but that's a `no-std` target, and I
suppose a lot of dependencies don't work that way (e.g.
https://github.com/gfx-rs/wgpu/issues/6826)
- Using the `-Ctarget-cpu=mvp` and `-Zbuild-std=panic_abort,std` flags,
and the `RUSTC_BOOTSTRAP=1` escape hatch to allow using the latter with
non-`nightly` toolchains - until
https://github.com/rust-lang/wg-cargo-std-aware is stabilized. (For
reference:
https://github.com/ruffle-rs/ruffle/pull/18528/files#diff-fb2896d189d77b35ace9a079c1ba9b55777d16e0f11ce79f776475a451b1825a)

I don't think either of these is particularly advantageous, so I suggest
just accepting that browsers will have to have some extensions
implemented to run `egui`.

examples/puffin_profiler/Cargo.toml

@@ -4,7 +4,7 @@ version = "0.1.0"
 authors = ["Emil Ernerfeldt <emil.ernerfeldt@gmail.com>"]
 license = "MIT OR Apache-2.0"
 edition = "2021"
-rust-version = "1.81"
+rust-version = "1.84"
 publish = false
 
 [package.metadata.cargo-machete]