docs: Add upgrading guidelines for CometBFT v0.34.27 (#381)

* docs: Add upgrading guidelines for CometBFT v0.34.27

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Expand on env vars fallback support

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* docs: Add upgrading guidelines to v0.34.x docs

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* docs: Update docs intro root with upgrading guidelines link

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Add note on Go module URL and historical upgrading instructions link

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* docs: Sync with upgrading root doc

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Update UPGRADING.md

* Adds a note about the use of go 1.19

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Signed-off-by: Thane Thomson <connect@thanethomson.com>
Co-authored-by: Lasaro <lasaro@informal.systems>