Core: `SecurityUtils.createMtlsKeyStore` should use a fixed algorithm for creating the `PrivateKey`

Callers of `SecurityUtils.createMtlsKeyStore` can direct the use of insecure algorithms in the construction of the mTLS connection. In general, we attempt to affix the algorithm used to inhibit use of algorithms with known vulnerabilities.

Personally, I would be fine with denying provided certs whose `cert.getPublicKey().getAlgorithm()` was `DH` or `DSA`, but in lieu of blocklisting known bad algorithms, perhaps it'd be more security affirmative to allowlist currently-OK algorithms, so we only ever use good versions, and suffer toil later if we have to retrofit additional entries.

My suggestion is to create a case statement just below where the method currently throws `IllegalArgumentException` for various cases, and add the set of known good algorithms, and throw an [`InvalidAlgorithmParameterException`](https://docs.oracle.com/en/java/javase/12/docs/api/java.base/java/security/InvalidAlgorithmParameterException.html) if it is not one of those. In addition, special case `DH` and `DSA` to give references to their vulnerabilities as described in [`InsecureCypherMode.identifyDiffieHellmanAndDsaVulnerabilities`](https://github.com/google/error-prone/blob/master/core/src/main/java/com/google/errorprone/bugpatterns/InsecureCipherMode.java).

/cc @sophieschmieg, as her guidance/requirements will be canonical for importing whatever changes are made here into the monorepo.

#### Environment details

1. API: Core
2. OS type and version: 5.17.6-1rodete1-amd64
3. Java version: OpenJDK Runtime Environment (build 11.0.13+8-google-release-451398016) OpenJDK 64-Bit Server VM (build 11.0.13+8-google-release-451398016, mixed mode, sharing)
4. version(s): ~head (with local patches)

#### Steps to reproduce

1. Add [errorprone](https://errorprone.info/) warnings to the build
2. Build.

`SecurityUtils.createMtlsKeyStore` will generate an [`InsecureCipherMode`](https://github.com/google/error-prone/blob/master/core/src/main/java/com/google/errorprone/bugpatterns/InsecureCipherMode.java) warning.

#### Code example

```java
PrivateKey key =
KeyFactory.getInstance(cert.getPublicKey().getAlgorithm()).generatePrivate(keySpecPKCS8);
```