Bump core-js from 3.9.0 to 3.15.2 in /backend

[dependabot]

Bumps core-js from 3.9.0 to 3.15.2.

Changelog

Sourced from core-js's changelog.

3.15.2 - 2021.06.29

• Worked around breakage related to zone.js loaded before core-js, #953
• Added NodeJS 16.4 -> Chrome 91 compat data mapping

3.15.1 - 2021.06.23

• Fixed cloning of regex through RegExp constructor, #948

3.15.0 - 2021.06.21

• Added RegExp named capture groups polyfill, #521, #944
• Added RegExp dotAll flag polyfill, #792, #944
• Added missed polyfills of Annex B features (required mainly for some non-browser engines), #336, #945:
  • escape
  • unescape
  • String.prototype.substr
  • Date.prototype.getYear
  • Date.prototype.setYear
  • Date.prototype.toGMTString
• Fixed detection of forbidden host code points in URL polyfill
• Allowed rhino target in core-js-compat / core-js-builder, added compat data for rhino 1.7.13, #942, thanks @​gausie
• .at marked as supported from FF90

3.14.0 - 2021.06.05

• Added polyfill of stable sort in { Array, %TypedArray% }.prototype.sort, #769, #941
• Fixed Safari 14.0- %TypedArray%.prototype.sort validation of arguments bug
• .at marked as supported from V8 9.2

3.13.1 - 2021.05.29

• Overwrites get-own-property-symbols third-party Symbol polyfill if it's used since it causes a stack overflow, #774
• Added a workaround of possible browser crash on Object.prototype accessors methods in WebKit ~ Android 4.0, #232

3.13.0 - 2021.05.26

• Accessible Object#hasOwnProperty (Object.hasOwn) proposal moved to the stage 3, May 2021 TC39 meeting

3.12.1 - 2021.05.09

• Fixed some cases of Function#toString with multiple core-js instances
• Fixed some possible String#split polyfill problems in V8 5.1

3.12.0 - 2021.05.06

• Added well-known symbol Symbol.metadata for decorators stage 2 proposal
• Added well-known symbol Symbol.matcher for pattern matching stage 1 proposal
• Fixed regression of V8 ~ Node 0.12 String(Symbol()) bug, #933

3.11.3 - 2021.05.05

• Native promise-based APIs Promise#{ catch, finally } returns polyfilled Promise instances when it's required

3.11.2 - 2021.05.03

• Added a workaround of WebKit ~ iOS 10.3 Safari Promise bug, #932
• Promise#then of incorrect native Promise implementations with correct subclassing no longer wrapped
• Changed the order of Promise feature detection, removed unhandled rejection tracking check in non-browser non-node platforms

... (truncated)

Commits

• ea6ee01 3.15.2
• 9125de6 working around breakage related to zone.js loaded before core-js, close #953
• 50b2157 update badges
• 560e783 update badges
• 7e9c305 update badges
• 90e84c8 Update internal-metadata.js
• 71a3baf Update internal-metadata.js
• d7409d1 3.15.1
• aa66fb2 fix cloning of regex through RegExp constructor, close #948
• 4f7f304 3.15.0
• Additional commits viewable in compare view

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting @dependabot rebase.

---

Dependabot commands and options

You can trigger Dependabot actions by commenting on this PR:

• @dependabot rebase will rebase this PR
• @dependabot recreate will recreate this PR, overwriting any edits that have been made to it
• @dependabot merge will merge this PR after your CI passes on it
• @dependabot squash and merge will squash and merge this PR after your CI passes on it
• @dependabot cancel merge will cancel a previously requested merge and block automerging
• @dependabot reopen will reopen this PR if it is closed
• @dependabot close will close this PR and stop Dependabot recreating it. You can achieve the same result by closing it manually
• @dependabot ignore this major version will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this minor version will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this dependency will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)