Preparing release 1.21.0

[joe-elliott]

Which problem is this PR solving?

• Prepares master for 1.21.0 release
• Fixes 1.21 release #2627

Short description of the changes

Updated the changelog in preparation for release. Per discussions in the bi-weekly we will not actually merge this PR and build images until early next week, but I wanted this up in case we needed to discuss the changelog.

TODO

• [ ] Merge Remove deprecated es.max-num-spans #2482 before this PR so it's in 1.21.0

Notes

• Since we are pinning jaeger-ui at a commit that is not a release I went ahead and included a jaeger-ui changelog as well. It appears that normally we just refer to the UI changelog, but that was not an option here
• I did not include the following lines in the changelog. Since this is my first time, I'm including them here in case we wanted to keep any of these:

* Fix flaky TestReload ([#2624](https://github.com/jaegertracing/jaeger/pull/2624), [@albertteoh](https://github.com/albertteoh))
* Fix flaky TestAddCertsToWatch_remove_ca test ([#2610](https://github.com/jaegertracing/jaeger/pull/2610), [@albertteoh](https://github.com/albertteoh))
* Remove md-to-godoc ([#2598](https://github.com/jaegertracing/jaeger/pull/2598), [@yurishkuro](https://github.com/yurishkuro))
* Clean-up error handling and logging ([#2590](https://github.com/jaegertracing/jaeger/pull/2590), [@tzzed](https://github.com/tzzed))
* Fix file descriptor leaks in tlscfg unit tests ([#2592](https://github.com/jaegertracing/jaeger/pull/2592), [@chlunde](https://github.com/chlunde))
* Exit properly after displayed help ([#2589](https://github.com/jaegertracing/jaeger/pull/2589), [@tzzed](https://github.com/tzzed))
* Improve `build-ui` Makefile target and fix dependencies ([#2573](https://github.com/jaegertracing/jaeger/pull/2573), [@albertteoh](https://github.com/albertteoh))
* Simplify test coverage collection ([#2560](https://github.com/jaegertracing/jaeger/pull/2560), [@PankhudiB](https://github.com/PankhudiB))
* Add mergify ([#2559](https://github.com/jaegertracing/jaeger/pull/2559), [@jpkrohling](https://github.com/jpkrohling))
* Replaced devbots with issue-labeler ([#2558](https://github.com/jaegertracing/jaeger/pull/2558), [@jpkrohling](https://github.com/jpkrohling))
* Align help message with default values ([#2553](https://github.com/jaegertracing/jaeger/pull/2553), [@tobiaskohlbau](https://github.com/tobiaskohlbau))
* Added .nocover to otel mains ([#2524](https://github.com/jaegertracing/jaeger/pull/2524), [@joe-elliott](https://github.com/joe-elliott))
* Adding missing build instruction #1887 ([#2542](https://github.com/jaegertracing/jaeger/pull/2542), [@listaction](https://github.com/listaction))
* README.md: Point to operator under deployment section ([#2539](https://github.com/jaegertracing/jaeger/pull/2539), [@lilic](https://github.com/lilic))
* Remove Go report card ([#2529](https://github.com/jaegertracing/jaeger/pull/2529), [@yurishkuro](https://github.com/yurishkuro))
* Fix max-num-spans flag's usage message ([#2528](https://github.com/jaegertracing/jaeger/pull/2528), [@albertteoh](https://github.com/albertteoh))

[codecov]

Codecov Report

Merging #2630 (0a48d9d) into master (1d22aaf) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2630      +/-   ##
==========================================
+ Coverage   95.05%   95.07%   +0.02%     
==========================================
  Files         210      210              
  Lines        9410     9410              
==========================================
+ Hits         8945     8947       +2     
+ Misses        388      387       -1     
+ Partials       77       76       -1     

Impacted Files	Coverage Δ
plugin/storage/badger/spanstore/reader.go	96.08% <0.00%> (+0.71%)	⬆️

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[yurishkuro]

@joe-elliott we should do a formal (numbered) release of the UI, to keep with the established process, rather than pointing to a specific commit.

[yurishkuro]

👍