Bump MSRV (Minimum Supported Rust Version) to 1.54.0 (#703)

1.51.0 clippy has been giving me trouble (not reporting all problems),
and so I take the easy way out and just bump MSRV.

We will upgrade to 1.56.0 once it comes around anyway
to get access to that sweet disjoint capture in closures
(https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html)

egui_web/src/shader/vertex_100es.glsl

@@ -1,31 +0,0 @@
-precision mediump float;
-uniform vec2 u_screen_size;
-attribute vec2 a_pos;
-attribute vec2 a_tc;
-attribute vec4 a_srgba;
-varying vec4 v_rgba;
-varying vec2 v_tc;
-
-// 0-1 linear  from  0-255 sRGB
-vec3 linear_from_srgb(vec3 srgb) {
-  bvec3 cutoff = lessThan(srgb, vec3(10.31475));
-  vec3 lower = srgb / vec3(3294.6);
-  vec3 higher = pow((srgb + vec3(14.025)) / vec3(269.025), vec3(2.4));
-  return mix(higher, lower, vec3(cutoff));
-}
-
-// 0-1 linear  from  0-255 sRGBA
-vec4 linear_from_srgba(vec4 srgba) {
-  return vec4(linear_from_srgb(srgba.rgb), srgba.a / 255.0);
-}
-
-void main() {
-  gl_Position = vec4(
-                     2.0 * a_pos.x / u_screen_size.x - 1.0,
-                     1.0 - 2.0 * a_pos.y / u_screen_size.y,
-                     0.0,
-                     1.0);
-  // egui encodes vertex colors in gamma spaces, so we must decode the colors here:
-  v_rgba = linear_from_srgba(a_srgba);
-  v_tc = a_tc;
-}