Replace "start" with "flex-start" in CSS (#2695)
Currently when using react-spectrum eslint will show a warning about
start having mixed support.
```
./node_modules/@react-spectrum/breadcrumbs/dist/main.css (./node_modules/css-loader/dist/cjs.js??ref--5-oneOf-4-1!./node_modules/postcss-loader/src??postcss!./node_modules/@rea
Warning
(1:745) start value has mixed support, consider using flex-start instead
./node_modules/@react-spectrum/breadcrumbs/dist/main.css (./node_modules/css-loader/dist/cjs.js??ref--5-oneOf-4-1!./node_modules/postcss-loader/src??postcss!./node_modules/@rea
Warning
(1:83) start value has mixed support, consider using flex-start instead
```
This fixes those warnings.
This takes over from https://github.com/adobe/react-spectrum/pull/1793,
which has been inactive since April 2021 and also borrows some changes
from https://github.com/adobe/react-spectrum/pull/686 that never got
applied.
A total of 5 files are modified:
```
$ git diff --stat main...HEAD
packages/@adobe/spectrum-css-temp/components/breadcrumb/index.css | 4 ++--
packages/@adobe/spectrum-css-temp/components/coachmark/index.css | 2 +-
packages/@adobe/spectrum-css-temp/components/dialog/index.css | 2 +-
packages/@adobe/spectrum-css-temp/components/menu/index.css | 6 +++---
packages/@adobe/spectrum-css-temp/components/sidenav/index.css | 2 +-
5 files changed, 8 insertions(+), 8 deletions(-)
```
Co-authored-by: Robert Snow <rsnow@adobe.com>
