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`.

.github/workflows/deploy_web_demo.yml

@@ -39,7 +39,7 @@ jobs:
         with:
           profile: minimal
           target: wasm32-unknown-unknown
-          toolchain: 1.81.0
+          toolchain: 1.84.0
           override: true
 
       - uses: Swatinem/rust-cache@v2

.github/workflows/preview_build.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.81.0
+          toolchain: 1.84.0
           targets: wasm32-unknown-unknown
       - uses: Swatinem/rust-cache@v2
         with:

.github/workflows/rust.yml

@@ -5,7 +5,7 @@ name: Rust
 env:
   RUSTFLAGS: -D warnings
   RUSTDOCFLAGS: -D warnings
-  NIGHTLY_VERSION: nightly-2024-09-11
+  NIGHTLY_VERSION: nightly-2025-04-22
 
 jobs:
   fmt-crank-check-test:
@@ -18,7 +18,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.81.0
+          toolchain: 1.84.0
 
       - name: Install packages (Linux)
         if: runner.os == 'Linux'
@@ -83,7 +83,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.81.0
+          toolchain: 1.84.0
           targets: wasm32-unknown-unknown
 
       - run: sudo apt-get update && sudo apt-get install libgtk-3-dev libatk1.0-dev
@@ -155,7 +155,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: EmbarkStudios/cargo-deny-action@v2
         with:
-          rust-version: "1.81.0"
+          rust-version: "1.84.0"
           log-level: error
           command: check
           arguments: --target  ${{ matrix.target }}
@@ -170,7 +170,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.81.0
+          toolchain: 1.84.0
           targets: aarch64-linux-android
 
       - name: Set up cargo cache
@@ -189,7 +189,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.81.0
+          toolchain: 1.84.0
           targets: aarch64-apple-ios
 
       - name: Set up cargo cache
@@ -208,7 +208,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.81.0
+          toolchain: 1.84.0
 
       - name: Set up cargo cache
         uses: Swatinem/rust-cache@v2
@@ -232,7 +232,7 @@ jobs:
           lfs: true
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.81.0
+          toolchain: 1.84.0
 
       - name: Set up cargo cache
         uses: Swatinem/rust-cache@v2