Update changelog and version ahead of v0.13.20 release

[jsirianni]

Description of Changes

• Added Azure Operators to change log
• Added PR link for Flatten operator in change log
• Bumped Stanza version to v0.13.20

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.637912	+0.20685768	125.45151	-0.8613968
1	5000	5.155343	-0.08573294	134.19235	+0.9520569
1	10000	10.569098	+0.10346413	141.09105	+0.53689575
1	50000	50.583786	+0.92832565	173.57031	-0.13537598
1	100000	96.5886	-2.143097	233.98936	+2.0074158
10	100	1.9138589	-0.10324144	130.2306	-0.7719574
10	500	6.2242813	-0.32764292	134.51225	-0.012390137
10	1000	12.362247	-0.20703125	145.03085	+2.9721222
10	5000	59.811718	+4.1408844	184.89345	+0.51345825
10	10000	108.534676	+5.0663605	226.04417	+9.871216

[codecov]

Codecov Report

Merging #290 (81080b4) into master (2673729) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
- Coverage   70.67%   70.62%   -0.05%     
==========================================
  Files         112      112              
  Lines        5956     5956              
==========================================
- Hits         4209     4206       -3     
- Misses       1300     1306       +6     
+ Partials      447      444       -3     

Impacted Files	Coverage Δ
operator/builtin/output/otlp/otlp.go	61.73% <0.00%> (-3.70%)	⬇️

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[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.5689933	+0.13793898	126.80334	+0.49043274
1	5000	5.327336	+0.08625984	132.06479	-1.1755066
1	10000	10.448521	-0.017113686	140.18616	-0.36799622
1	50000	50.29359	+0.6381302	175.03395	+1.3282623
1	100000	104.414986	+5.6832886	233.07126	+1.0893097
10	100	1.948357	-0.06874335	129.02815	-1.974411
10	500	6.3277597	-0.22416449	136.06183	+1.5371857
10	1000	12.293409	-0.27586937	143.07542	+1.0166931
10	5000	58.96598	+3.295147	187.14467	+2.764679
10	10000	109.542786	+6.0744705	221.96848	+5.795517