chore: update lru-cache dependency, update Node versions in CI

[JKRhb]

Spin-off from #373 that upgrades the lru-cache, because it is a bit tedious to debug otherwise.

Also updates the Node versions used by the CI, since the most recent lru-cache version seems to be incompatible with Node 14. Since Node 14 is EOL by way, applying this update might be overdue anyway.

[github-actions]

Pull Request Test Coverage Report for Build 6920889522

• 12 of 14 (85.71%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 91.757%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/server.ts	12	14	85.71%

Files with Coverage Reduction	New Missed Lines	%
lib/server.ts	3	86.75%

Totals
Change from base Build 5048002328:	-0.1%
Covered Lines:	2857
Relevant Lines:	3072

---

💛 - Coveralls

[github-actions]

Pull Request Test Coverage Report for Build 6941605775

• 13 of 14 (92.86%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 91.871%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/server.ts	13	14	92.86%

Totals
Change from base Build 5048002328:	-0.02%
Covered Lines:	2861
Relevant Lines:	3072

---

💛 - Coveralls

[Apollon77]

Al right ... makes sense, BUT you also need to change https://github.com/coapjs/node-coap/blob/master/package.json#L68 !

[JKRhb]

Al right ... makes sense, BUT you also need to change https://github.com/coapjs/node-coap/blob/master/package.json#L68 !

Thank you, good catch! :) I now set the minimum version to 18, by the way, as I remembered that 16 was phased out early due to the EOL of OpenSSL 1.1.1

[JKRhb]

If the current changes still work for you, then you could just hit the merge button :)