misc/ruby-bundler: update to 2.2.8

2.2.8 (2021-02-02)

Enhancements:
* Add a CHANGELOG.md file to gems generated by bundle gem #4093
* Support gemified set #4297

Bug fixes:
* Fix standalone Kernel.require visibility #4337

Performance:
* Fix resolver edge cases and speed up bundler #4277

2.2.7 (2021-01-27)

Enhancements:
* Improve error messages when dependency on bundler conflicts with running
  version #4308
* Avoid showing platforms with requirements in error messages #4310
* Introduce disable_local_revision_check config #4237
* Reverse rubygems require mixin with bundler standalone #4299

Bug fixes:
* Fix releasing from a not yet pushed branch #4309
* Install cache only once if it already exists #4304
* Fix force_ruby_platform no longer being respected #4302

Performance:
* Fix resolver dependency comparison #4289

(taca)