Backport of 'Pin the Baseline date in .browserslistrc' into 19.4

What does this MR do and why?

This backports the two commits of !261541 (merged) so that the compiled assets of 19.4 no longer depend on the day of the build:

• Pin the Baseline date in .browserslistrc. On this branch's lockfile, the date 2026-10-01 resolves to the same browsers as the undated query does today, so the compiled assets do not change.
• Add .browserslistrc to the assets cache key. This forces one recompile, so the pin reaches the assets.

The third commit of that merge request, which adds a keep that moves the Baseline date monthly, is not backported. 19.4 keeps the 2026-10-01 browser set.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.