fix: re-build production bundle if index.html changes

[mcollovati]

Stores index.html hash in stats.json and forces production bundle to be re-built if file contents have changed.
Changes to index.html do not trigger a dev bundle re-generation since in dev mode the file is served directly from the frontend folder.

Fixes #20629

[mcollovati]

If this change gets merged, we need to check and potentially update the index.html file in starters to match the default contents, to prevent a custom production bundle to be generated if not needed.

For example, skeleton-starter-flow-spring contains a version of index.html whose only difference with default is a removal of a comment.

https://github.com/vaadin/skeleton-starter-flow-spring/blob/v24/src/main/frontend/index.html

[github-actions]

Test Results

1 159 files  ± 0  1 159 suites  ±0   1h 31m 8s ⏱️ - 3m 49s
7 568 tests + 6  7 512 ✅ + 4  56 💤 +2  0 ❌ ±0 
7 917 runs   - 10  7 852 ✅  - 12  65 💤 +2  0 ❌ ±0 

Results for commit ae55f2e. ± Comparison against base commit 1b58bd5.

♻️ This comment has been updated with latest results.

[sonarcloud]

Quality Gate passed

Issues
151 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mcollovati]

@caalador I was thinking if we should remove indentation and comments before hashing, to prevent irrelevant changes causing a bundle rebuild. What do you think?

[caalador]

@caalador I was thinking if we should remove indentation and comments before hashing, to prevent irrelevant changes causing a bundle rebuild. What do you think?

removing comments might be a bit too much for running the check and hash creation. trimming might be fine, but it will make it more complex than now when we only read and hash.
Perhaps keep it in mind and add it if someone has a problem big enough that they all the time meet the issue with whitespace or comment updates.